Bloods can show little to a lot of change over the 1st 3 years where it generally settles down. It varies from snake to snake but in my experience in general there is not a huge amount of change.
As far as their heads go I don't think it is known why although various speculation exists including for mating purposes etc.
All I know is that they have the ability to alter the color or shade of their heads as I have seen it many times over the20 odd years of keeping snakes and being around them. And I have seen it throughout the year not just in breeding season etc
Maybe someone else has a more scientific answer but as far as I know there isn't one.