I wonder if we shouldn't do it the other way around: color specifically only *allowed* `LabelDefs`. AFAIK, the only labels that we should color ar recursive tail calls, since they appear as method calls in source code but they get back to us as labels. 

At any rate, we should also skip `do..while`, as they have a different prefix and are [translated to labels](https://github.com/scala/scala/blob/master/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala#L1316) too.