*Do note that this would only apply to blocks & mobs, not items in your inventory.
To give the game more "life", I think that certain blocks and mobs should be able to exist in different colors, just like grass and leaves do. The color would be chosen at random from a color palette, sort of like the biome palette grass and leaves use. Here is an example using pigs and mushrooms (scratch the white mushroom):
Animal breeding could work like this: two points get placed on the color palette; one for the father, another for the mother. The child mob would be a random color found on the line between the two points. Selective breeding ftw.
If you want to support this, make sure to:
I've also created some support banners for signatures (read spoiler):