It is depigmentated. The undertail being white is a common MODO phenotype but not the undertail coverts as well. Additionally, you’ll notice the entire tail is white when normally pigmented doves have black tips to rectrices.
I have two though iI didn’t have my camera for either of them 
the first one was of a black-backed gull (Larus marinus) that had several white feathers along its wing which are normally black.
The second of was of a pure black pigeon.
When I was doing my master’s thesis, I had thousands of Southern toad Anaxyrus terrestris tadpoles. Normally, these are very dark brown, but I noticed that a small percentage of them were a golden color. My professor asked me to make a note of the numbers of each, because he had never seen that before. When they emerged as metamorphs, the normal dusky tadpoles were normal brown toads, but the golden tadpoles were beige toads. Raising them to adulthood was beyond the scope of my project, but it would have been interesting to see if they retained their color differences.
Now, that is something. You must have found it right after it molted, before the new exoskeleton hardened.
Can’t you just search for any observation tagged “albinistic” or “leucistic”?
That isn’t the only possibility of an unusual color.
i’ll contribute this erythristic partridge planthopper nymph! exciting find for me
i made this thread inspired by all of the colour ones, and this is for any unique colours unusual to the species, not just those two ^w^
I merged the above thread into this previous one and reopened since they have the same topics.
I’ll have to see if i can get a picture, but there is a squirrel (not sure if its a fox squirrel or a grey squirrel) that i call Maple that visits my yard that has a striped tail, in rings almost like a raccoon