This uses hsc2hs. As far as I can tell, alignment is unused. I've never had an error from using 'undefined' there.
#include "c_interface.h"
instance Storable Color where sizeOf _ = #size Color alignment _ = undefined peek = peek_color poke = poke_color
peek_color colorp = do r <- (#peek Color, r) colorp :: IO CUChar g <- (#peek Color, g) colorp :: IO CUChar b <- (#peek Color, b) colorp :: IO CUChar a <- (#peek Color, a) colorp :: IO CUChar return $ Color (d r) (d g) (d b) (d a) where d uchar = fromIntegral uchar / 255.0
poke_color colorp (Color r g b a) = do (#poke Color, r) colorp (c r) (#poke Color, g) colorp (c g) (#poke Color, b) colorp (c b) (#poke Color, a) colorp (c a) where c double = Util.c_uchar (floor (double*255))