Probably 2 different anodizing batches in that picture...or the pistons on the outside had a different level of exposure to something. Heat/UV/cleaning chemicals can all fade anodizing dyes...while you can do UV resistant black (that's why most automatic doors at grocery stores, etc..are black)...nobody is gonna waste money on that for caliper pistons as it makes no difference.
My pistons are all hard anodized. The coloration can vary a bit, but the color is insignificant. I have pistons that are black (color in anodizing is just a dye) and pistons that are bright silver and pistons that are somewhere in between....they are all hard anodized. I don't spec the color because I don't care...as long as the properties are there.