MOB-2275 Calculate background color for map tiles' sets

@aquilax I was just lazy and I wanted to write a method call instead of writing a loop with conditions or something :japanese_ogre: 
@hakubo our JSHint/JSLint tools we use complained when I was using `_` and I was lazy again and changed `_` to `underscore` instead of investigating why we have such a check in pre-commit hooks' tools