The cherry grove biome appears to either have hardcoded grass or foliage colors or pulls the values from somewhere else. The grass.png and foliage.png files in resource packs don't influence the color at all.

The attached screenshots both show the border between a plains biome (left) and a cherry grove biome (right).  In the screenshot where all the grass is green, it's using the normal grass.png from the current resource pack. In the second screenshot, it's using a grass.png that's been inverted to shades of purple. The plains biome exhibits the expected behavior and the grass is purple, but the cherry grove biome's grass stays the same shade of green as the first screenshot. Foliage in the cherry grove biome similarly ignores foliage.png.

Just in case, I also tried this with a resource pack that only contains a grass.png file in the colormaps directory, with the same result, so this is not isolated to a particular resource pack.