When you pass a yellow signal and the next one is dim or even dark you have to assume it is red until you are certain it is not. So a dim signal, while inconvenient, should never lead to someone running past it because they didn't see it. The combination of a dim red and sunlit green however could be a recipe for disaster.

I've stopped after passing a snow covered signal (the previous signal was green) since by rule a signal that is improperly displayed (dark, obstructed etc.) must be regarded as the most restrictive indication that signal can display which is usually red.