Pretty much any color is possible except for white. White to my knowledge is not currently possible… anodized yoyos that look white are just anodized raw/silver.
Handling colors does vary from color to color, but I wouldn’t necessarily say one color is harder than another. The difficulty in anodizing comes from the layers/patterns/stencils, etc. People often think that fancy splashes are hard, but they really aren’t, because they hide ano flaws. The hardest colorways to execute are complex stencils/layers on top of a solid background. For example, these were the hardest for me:
