I haven't seen many pure white A&M, most of them have some color on them somewhere. They were bred to have white breast meat, I don't think it was their whole bodies that were meant to be white. I could be wrong about that, but this is my take on it.
If you want to have birds with as little color as possible, I would keep only those that are pure white, or those with the smallest spots on their heads, as this is acceptable for A&Ms. Body spots shouldn't be evident, though most of mine do have them.
I haven't processed any of mine, so I can't say about the 'all white meat'.