The biome-specific coloration of foliage and grass fades when near the world border of the 'old' world type, possibly due to a lack of a biome ID past the wall. It can be observed in all biomes, though in most it is only barely perceptible. Making a world infinite does not have any effect on the fading.
I tested this using pre-0.9.0 worlds as well as the current version's 'old' world type. The attached photos show the world border next to a swampland, where the issue is most prominent.