Yellow consits of closely equal portions of Red+Green (On a RGB tube that is

Which pots are you fiddling with? There are different pots that do different things on each monitor.
Some are "Drive" pots that affect the overall brightness of the gun and others are "cutoff" pots that affect the minimum signal level it takes for the gun to turn on.
Each control the picture differently.
It's strange that you say all the other colors look good, since Yellow is a simple combination of 2 of the base colors. It's usally a core color (RGB) that is missing.
Is your monitor magnetized? (You say the color is off)... is the degaussing circuit working correctly?