The Meaning of Life, the Universe, and Everything.
Your side block file is fine, the issue is with your overlay. The overlay file uses the colormap for the biomes in order to change the color of grass slightly from from biome to biome. To do this best you want to overlay only the portion of the side of your block that is actually grass and then you want to make it a grayscale image instead of an RGB or indexed color. Like dyed leather armor the game uses the shades of the grayscale to change the shade of the overlaying color- your issue is your overlay file is the entirety of the block and is colored so the colormap isn't sure exactly how to treat it.