The bug
Some naturally spawned animals always spawn with the default color / variant.
Horse: white
Llama: creamy
Sheep: white
Rabbit: brown
Parrot: red
Spawning them using spawn eggs or /summon
is not affected. Apparently some values are still random, like for example the health of the horses.
Does not affect superflat worlds.
Linked issues
is duplicated by
Attachments
Comments

Apparently affects llamas and sheep as well, included them therefore