Ahh good good! I'm thinking the Sun Radiance brightens the color of the sky around it which is why the file for the default Sun Radiance is grayscale.
In the sky.material file, the SunRadiance material has a scene_blend add pass:
"scene_blend add: The colour of the rendering output is added to the scene. Good for explosions, flares, lights, ghosts etc." -Ogre Material manual
From what I understand from that is the grayscale lighter shades (like white and light grays) are "added" ontop of the blue sky to make it appear brighter or appear like the sun is glowing, as it does. I hope that makes sense. ^^