diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,1 +1,3 @@
 # union-color
+
+https://hackage.haskell.org/package/union-color
diff --git a/src/Data/Color.hs b/src/Data/Color.hs
--- a/src/Data/Color.hs
+++ b/src/Data/Color.hs
@@ -2,26 +2,53 @@
 {-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
 
 module Data.Color (
-	-- * Alpha
+
+	-- * ALPHA
+
 	Alpha, pattern AlphaWord8, pattern AlphaWord16, pattern AlphaWord32,
 	pattern AlphaInt32, alphaInt32,
 	pattern AlphaDouble, alphaDouble, alphaRealToFrac,
+
 	-- * RGB
+
 	Rgb, pattern RgbWord8, pattern RgbWord16, pattern RgbWord32,
 	pattern RgbInt32, rgbInt32,
 	pattern RgbDouble, rgbDouble, rgbRealToFrac,
+
 	-- * RGBA
+
 	-- ** Straight
+
 	Rgba, pattern RgbaWord8, pattern RgbaWord16, pattern RgbaWord32,
 	pattern RgbaInt32, rgbaInt32,
 	pattern RgbaDouble, rgbaDouble,
+
 	-- ** Premultiplied
+
 	pattern RgbaPremultipliedWord8, rgbaPremultipliedWord8,
 	pattern RgbaPremultipliedWord16, rgbaPremultipliedWord16,
 	pattern RgbaPremultipliedDouble, rgbaPremultipliedDouble,
+
 	-- ** From and To Rgb and Alpha
+
 	toRgba, fromRgba,
+
 	-- ** Convert Fractional
-	rgbaRealToFrac ) where
 
+	rgbaRealToFrac,
+
+	-- * GRAY SCALE
+
+	Gray,
+	pattern GrayWord1, grayWord1, pattern GrayWord2, grayWord2,
+	pattern GrayWord4, grayWord4,
+	pattern GrayWord8, pattern GrayWord16, pattern GrayWord32,
+	pattern GrayInt32,
+	pattern GrayDouble, grayDouble
+
+	-- * GRAY SCALE WITH ALPHA
+
+	) where
+
 import Data.Color.Internal
+import Data.Color.Internal.GrayScale
diff --git a/src/Data/Color/Internal/GrayScale.hs b/src/Data/Color/Internal/GrayScale.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Color/Internal/GrayScale.hs
@@ -0,0 +1,289 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
+{-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
+
+module Data.Color.Internal.GrayScale where
+
+import Data.Bits
+import Data.Word
+import Data.Int
+
+import Data.Color.Internal
+
+data Gray d
+	= GrayWord1_ Word8 | GrayWord2_ Word8 | GrayWord4_ Word8
+	| GrayWord8_ Word8 | GrayWord16_ Word16 | GrayWord32_ Word32
+	| GrayInt32_ Int32 | GrayDouble_ d
+	deriving Show
+
+{-# COMPLETE GrayWord1 #-}
+
+pattern GrayWord1 :: RealFrac d => Word8 -> Gray d
+pattern GrayWord1 g <- (grayToWord1 -> g)
+
+grayWord1 :: Word8 -> Maybe (Gray d)
+grayWord1 w
+	| 0 <= w && w < 2 = Just $ GrayWord1_ w
+	| otherwise = Nothing
+
+grayToWord1 :: RealFrac d => Gray d -> Word8
+grayToWord1 (GrayWord1_ g) = g
+grayToWord1 (GrayWord2_ g) = g `shiftR` 1
+grayToWord1 (GrayWord4_ g) = g `shiftR` 3
+grayToWord1 (GrayWord8_ g) = g `shiftR` 7
+grayToWord1 (GrayWord16_ g) = fromIntegral $ g `shiftR` 15
+grayToWord1 (GrayWord32_ g) = fromIntegral $ g `shiftR` 31
+grayToWord1 (GrayInt32_ g) = fromIntegral $ g `shiftR` 30
+grayToWord1 (GrayDouble_ g) = fracToWord8 g `shiftR` 7
+
+{-# COMPLETE GrayWord2 #-}
+
+pattern GrayWord2 :: RealFrac d => Word8 -> Gray d
+pattern GrayWord2 g <- (grayToWord2 -> g)
+
+grayWord2 :: Word8 -> Maybe (Gray d)
+grayWord2 g
+	| 0 <= g && g < 4 = Just $ GrayWord2_ g
+	| otherwise = Nothing
+
+grayToWord2 :: RealFrac d => Gray d -> Word8
+grayToWord2 (GrayWord1_ g) = g `shiftL` 1 .|. g
+grayToWord2 (GrayWord2_ g) = g
+grayToWord2 (GrayWord4_ g) = g `shiftR` 2
+grayToWord2 (GrayWord8_ g) = g `shiftR` 6
+grayToWord2 (GrayWord16_ g) = fromIntegral $ g `shiftR` 14
+grayToWord2 (GrayWord32_ g) = fromIntegral $ g `shiftR` 30
+grayToWord2 (GrayInt32_ g) = fromIntegral $ g `shiftR` 29
+grayToWord2 (GrayDouble_ g) = fracToWord8 g `shiftR` 6
+
+{-# COMPLETE GrayWord4 #-}
+
+pattern GrayWord4 :: RealFrac d => Word8 -> Gray d
+pattern GrayWord4 g <- (grayToWord4 -> g)
+
+grayWord4 :: Word8 -> Maybe (Gray d)
+grayWord4 g
+	| 0 <= g && g < 16 = Just $ GrayWord4_ g
+	| otherwise = Nothing
+
+grayToWord4 :: RealFrac d => Gray d -> Word8
+grayToWord4 (GrayWord1_ g) =
+	g `shiftL` 3 .|. g `shiftL` 2 .|. g `shiftL` 1 .|. g
+grayToWord4 (GrayWord2_ g) = g `shiftL` 2 .|. g
+grayToWord4 (GrayWord4_ g) = g
+grayToWord4 (GrayWord8_ g) = g `shiftR` 4
+grayToWord4 (GrayWord16_ g) = fromIntegral $ g `shiftR` 12
+grayToWord4 (GrayWord32_ g) = fromIntegral $ g `shiftR` 28
+grayToWord4 (GrayInt32_ g) = fromIntegral $ g `shiftR` 27
+grayToWord4 (GrayDouble_ g) = fracToWord8 g `shiftR` 4
+
+{-# COMPLETE GrayWord8 #-}
+
+pattern GrayWord8 :: RealFrac d => Word8 -> Gray d
+pattern GrayWord8 x <- (fromGrayWord8 -> x) where
+	GrayWord8 = GrayWord8_
+
+fromGrayWord8 :: RealFrac d => Gray d -> Word8
+fromGrayWord8 = \case
+	GrayWord1_ x ->
+		x `shiftL` 7 .|. x `shiftL` 6 .|.
+		x `shiftL` 5 .|. x `shiftL` 4 .|.
+		x `shiftL` 3 .|. x `shiftL` 2 .|.
+		x `shiftL` 1 .|. x
+	GrayWord2_ x ->
+		x `shiftL` 6 .|. x `shiftL` 4 .|.
+		x `shiftL` 2 .|. x
+	GrayWord4_ x -> x `shiftL` 4 .|. x
+	GrayWord8_ x -> x
+	GrayWord16_ x -> fromIntegral $ x `shiftR` 8
+	GrayWord32_ x -> fromIntegral $ x `shiftR` 24
+	GrayInt32_ x -> fromIntegral $ x `shiftR` 23
+	GrayDouble_ x -> fracToWord8 x
+
+fracToWord8 :: RealFrac d => d -> Word8
+fracToWord8 = round . (* 0xff)
+
+{-# COMPLETE GrayWord16 #-}
+
+pattern GrayWord16 :: RealFrac d => Word16 -> Gray d
+pattern GrayWord16 x <- (grayToWord16 -> x) where
+	GrayWord16 = GrayWord16_
+
+grayToWord16 :: RealFrac d => Gray d -> Word16
+grayToWord16 = \case
+	GrayWord1_ x ->
+		x' `shiftL` 15 .|. x' `shiftL` 14 .|.
+		x' `shiftL` 13 .|. x' `shiftL` 12 .|.
+		x' `shiftL` 11 .|. x' `shiftL` 10 .|.
+		x' `shiftL` 9 .|. x' `shiftL` 8 .|.
+		x' `shiftL` 7 .|. x' `shiftL` 6 .|.
+		x' `shiftL` 5 .|. x' `shiftL` 4 .|.
+		x' `shiftL` 3 .|. x' `shiftL` 2 .|.
+		x' `shiftL` 1 .|. x'
+		where x' = fromIntegral x
+	GrayWord2_ x ->
+		x' `shiftL` 14 .|. x' `shiftL` 12 .|.
+		x' `shiftL` 10 .|. x' `shiftL` 8 .|.
+		x' `shiftL` 6 .|. x' `shiftL` 4 .|.
+		x' `shiftL` 2 .|. x'
+		where x' = fromIntegral x
+	GrayWord4_ x ->
+		x' `shiftL` 12 .|. x' `shiftL` 8 .|. x' `shiftL` 4 .|. x'
+		where x' = fromIntegral x
+	GrayWord8_ x -> x' `shiftL` 8 .|. x' where x' = fromIntegral x
+	GrayWord16_ x -> x
+	GrayWord32_ x -> fromIntegral $ x `shiftR` 16
+	GrayInt32_ x -> fromIntegral $ x `shiftR` 15
+	GrayDouble_ x -> fracToWord16 x
+
+fracToWord16 :: RealFrac d => d -> Word16
+fracToWord16 = round . (* 0xffff)
+
+{-# COMPLETE GrayWord32 #-}
+
+pattern GrayWord32 :: RealFrac d => Word32 -> Gray d
+pattern GrayWord32 x <- (grayToWord32 -> x) where
+	GrayWord32 = GrayWord32_
+
+grayToWord32 :: RealFrac d => Gray d -> Word32
+grayToWord32 = \case
+	GrayWord1_ x ->
+		x' `shiftL` 31 .|. x' `shiftL` 30 .|.
+		x' `shiftL` 29 .|. x' `shiftL` 28 .|.
+		x' `shiftL` 27 .|. x' `shiftL` 26 .|.
+		x' `shiftL` 25 .|. x' `shiftL` 24 .|.
+		x' `shiftL` 23 .|. x' `shiftL` 22 .|.
+		x' `shiftL` 21 .|. x' `shiftL` 20 .|.
+		x' `shiftL` 19 .|. x' `shiftL` 18 .|.
+		x' `shiftL` 17 .|. x' `shiftL` 16 .|.
+		x' `shiftL` 15 .|. x' `shiftL` 14 .|.
+		x' `shiftL` 13 .|. x' `shiftL` 12 .|.
+		x' `shiftL` 11 .|. x' `shiftL` 10 .|.
+		x' `shiftL` 9 .|. x' `shiftL` 8 .|.
+		x' `shiftL` 7 .|. x' `shiftL` 6 .|.
+		x' `shiftL` 5 .|. x' `shiftL` 4 .|.
+		x' `shiftL` 3 .|. x' `shiftL` 2 .|.
+		x' `shiftL` 1 .|. x'
+		where x' = fromIntegral x
+	GrayWord2_ x ->
+		x' `shiftL` 30 .|. x' `shiftL` 28 .|.
+		x' `shiftL` 26 .|. x' `shiftL` 24 .|.
+		x' `shiftL` 22 .|. x' `shiftL` 20 .|.
+		x' `shiftL` 18 .|. x' `shiftL` 16 .|.
+		x' `shiftL` 14 .|. x' `shiftL` 12 .|.
+		x' `shiftL` 10 .|. x' `shiftL` 8 .|.
+		x' `shiftL` 6 .|. x' `shiftL` 4 .|.
+		x' `shiftL` 2 .|. x'
+		where x' = fromIntegral x
+	GrayWord4_ x ->
+		x' `shiftL` 28 .|. x' `shiftL` 24 .|.
+		x' `shiftL` 20 .|. x' `shiftL` 16 .|.
+		x' `shiftL` 12 .|. x' `shiftL` 8 .|.
+		x' `shiftL` 4 .|. x'
+		where x' = fromIntegral x
+	GrayWord8_ x ->
+		x' `shiftL` 24 .|. x' `shiftL` 16 .|. x' `shiftL` 8 .|. x'
+		where x' = fromIntegral x
+	GrayWord16_ x -> x' `shiftL` 16 .|. x' where x' = fromIntegral x
+	GrayWord32_ x -> x
+	GrayInt32_ x -> fromIntegral x `shiftL` 1
+	GrayDouble_ x -> fracToWord32 x
+
+fracToWord32 :: RealFrac d => d -> Word32
+fracToWord32 = round . (* 0xffffffff)
+
+{-# COMPLETE GrayInt32 #-}
+
+pattern GrayInt32 :: RealFrac d => Int32 -> Gray d
+pattern GrayInt32 x <- (grayToInt32 -> x) where
+	GrayInt32 = GrayInt32_
+
+grayToInt32 :: RealFrac d => Gray d -> Int32
+grayToInt32 = \case
+	GrayWord1_ x ->
+		x' `shiftL` 30 .|. x' `shiftL` 29 .|.
+		x' `shiftL` 28 .|. x' `shiftL` 27 .|.
+		x' `shiftL` 26 .|. x' `shiftL` 25 .|.
+		x' `shiftL` 24 .|. x' `shiftL` 23 .|.
+		x' `shiftL` 22 .|. x' `shiftL` 21 .|.
+		x' `shiftL` 20 .|. x' `shiftL` 19 .|.
+		x' `shiftL` 18 .|. x' `shiftL` 17 .|.
+		x' `shiftL` 16 .|. x' `shiftL` 15 .|.
+		x' `shiftL` 14 .|. x' `shiftL` 13 .|.
+		x' `shiftL` 12 .|. x' `shiftL` 11 .|.
+		x' `shiftL` 10 .|. x' `shiftL` 9 .|.
+		x' `shiftL` 8 .|. x' `shiftL` 7 .|.
+		x' `shiftL` 6 .|. x' `shiftL` 5 .|.
+		x' `shiftL` 4 .|. x' `shiftL` 3 .|.
+		x' `shiftL` 2 .|. x' `shiftL` 1 .|. x'
+		where x' = fromIntegral x
+	GrayWord2_ x ->
+		x' `shiftL` 29 .|. x' `shiftL` 27 .|.
+		x' `shiftL` 25 .|. x' `shiftL` 23 .|.
+		x' `shiftL` 21 .|. x' `shiftL` 19 .|.
+		x' `shiftL` 17 .|. x' `shiftL` 15 .|.
+		x' `shiftL` 13 .|. x' `shiftL` 11 .|.
+		x' `shiftL` 9 .|. x' `shiftL` 7 .|.
+		x' `shiftL` 5 .|. x' `shiftL` 3 .|.
+		x' `shiftL` 1 .|. x' `shiftR` 1
+		where x' = fromIntegral x
+	GrayWord4_ x ->
+		x' `shiftL` 27 .|. x' `shiftL` 23 .|.
+		x' `shiftL` 19 .|. x' `shiftL` 15 .|.
+		x' `shiftL` 11 .|. x' `shiftL` 7 .|.
+		x' `shiftL` 3 .|. x' `shiftR` 1
+		where x' = fromIntegral x
+	GrayWord8_ x ->
+		x' `shiftL` 23 .|. x' `shiftL` 15 .|.
+		x' `shiftL` 7 .|. x' `shiftR` 1
+		where x' = fromIntegral x
+	GrayWord16_ x -> x' `shiftL` 15 .|. x' `shiftR` 1
+		where x' = fromIntegral x
+	GrayWord32_ x -> fromIntegral $ x `shiftR` 1
+	GrayInt32_ x -> x
+	GrayDouble_ x -> fracToInt32 x
+
+fracToInt32 :: RealFrac d => d -> Int32
+fracToInt32 = round . (* 0x7fffffff)
+
+{-# COMPLETE GrayDouble #-}
+
+pattern GrayDouble :: RealFrac d => d -> Gray d
+pattern GrayDouble x <- (grayToFrac -> x)
+
+grayDouble :: (Ord d, Num d) => d -> Maybe (Gray d)
+grayDouble x
+	| 0 <= x && x <= 1 = Just $ GrayDouble_ x
+	| otherwise = Nothing
+
+grayToFrac :: (Eq d, Fractional d) => Gray d -> d
+grayToFrac = \case
+	GrayWord1_ x -> word1ToFrac x
+	GrayWord2_ x -> word2ToFrac x
+	GrayWord4_ x -> word4ToFrac x
+	GrayWord8_ x -> word8ToFrac x
+	GrayWord16_ x -> word16ToFrac x
+	GrayWord32_ x -> word32ToFrac x
+	GrayInt32_ x -> int32ToFrac x
+	GrayDouble_ x -> x
+
+word1ToFrac :: Fractional d => Word8 -> d
+word1ToFrac = fromIntegral
+
+word2ToFrac :: Fractional d => Word8 -> d
+word2ToFrac = (/ 0x3) . fromIntegral
+
+word4ToFrac :: Fractional d => Word8 -> d
+word4ToFrac = (/ 0xf) . fromIntegral
+
+word8ToFrac :: Fractional d => Word8 -> d
+word8ToFrac = (/ 0xff) . fromIntegral
+
+word16ToFrac :: Fractional d => Word16 -> d
+word16ToFrac = (/ 0xffff) . fromIntegral
+
+word32ToFrac :: Fractional d => Word32 -> d
+word32ToFrac = (/ 0xffffffff) . fromIntegral
+
+int32ToFrac :: Fractional d => Int32 -> d
+int32ToFrac = (/ 0x7fffffff) . fromIntegral
diff --git a/union-color.cabal b/union-color.cabal
--- a/union-color.cabal
+++ b/union-color.cabal
@@ -1,11 +1,11 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.35.2.
+-- This file has been generated from package.yaml by hpack version 0.38.1.
 --
 -- see: https://github.com/sol/hpack
 
 name:           union-color
-version:        0.1.2.1
+version:        0.1.3.0
 synopsis:       Unify verious color formats
 description:    Please see the README on GitHub at <https://github.com/YoshikuniJujo/union-color#readme>
 category:       data, graphics
@@ -29,6 +29,7 @@
   exposed-modules:
       Data.Color
       Data.Color.Internal
+      Data.Color.Internal.GrayScale
   other-modules:
       Paths_union_color
   hs-source-dirs: