It's not red-blue-yellow, it's RGB (red-green-blue). An easy way to access the RGB-cycle is to open up mspaint, double-click on a color to bring up the color picker, and then click "Define Custom Colors". The values on the right side, the RGB ones, are the ones you'll need to set the fog color. I recommend using more bleach-ish colors, so pick colors near the grey area to make them less bright.
Dark, yellow fog could be set to something like; R=125 G=120 B=60.
By the way, the RGB colors can have values from 0 to 255 only.
Your big has been erased by a signature because it was too mod.