Hmm I noticed, that only the priority is major for the color. Why isn't the severity as major as the priority? If I am a developer, I would notice the red ones first, but they may have a trivial severity ... the criticals with a lower priority you wouldn't notice as good as the ones with high priority.
Sry for bad English, I'm German ;)
The color is based on Priority, because well, its the Priority. The severity is basically to say how severe it is.
If something is of the highest severity, set the priority to the highest.