I apologize if this has been reported before; I was unable to find a credible solution to this issue.
When playing Redpower 4.3.4 #29 and 4.3.7 #32 1.6.4, the sky turns dark when the HUD is disabled. No error logs whatsoever.

I've tried removing all other mods and running with just what is required to run Redpower(with all modules), but the issue still persists.

Sky related issues seem to be re-occurring. Just search for sky issues.
Often it is some incompatibility with another mod e.g Optifine or other graphical mods.
Sometimes switching from 'Fancy' to 'Fast' graphics fixes the problem.

This sounds similar to when I had shaders mod and 3rd person view was pretty dark..

since removing I haven't had it happen again. Do you use shaders?