If it looked like a Grackle and moved like a Grackle then it's likely that's what it was.
Search for "Leucistic Grackle". They do exist.
Leucism is a condition causing partial colour pigmentation loss and isn't exclusive to birds.
The collar you mention is just the normal irridescence found around the neck area but it will appear exagerated against the unusually pale head feathers. It's not surprising it caught your eye.