The soldermask color is purely a function of designer preference, and whether the person paying for the boards is willing to foot the bill for custom soldermask colors. It has no relation to whether the board is ROHS compliant or anything else.
Green is the most common just because it's a defacto standard. You can get ROHS and non-ROHS green boards, as you can with any other coloring.
It's worth noting that it's entirely possible that that assembler only had an in-house standard where they ordered boards that needed to be ROHS compliant with a blue soldermask, and boards that didn't need to be ROHS with green (or any other color) soldermask. However, that would be a function of that assembler in particular, not an industry wide thing.