I'm using Photoshop CS4 and have a project where I'm using a layer/section of brushed chrome and another adjacent layer/section in charcoal carbon fiber (pattern) background. On the CF layer, I'm adding a text layer which is in color. If I start the project in RGB mode, it prints out with a purple cast on both the metal and the CF. If I switch to a Greyscale mode it prints out fine, but now I can't use the red colored text which was created in RGB mode. How do I add RGB elements (layers) to my Greyscale layered image?
I think kamalo gives good advice, but "prints out with [color cast]" implies that the printer might be the culprit here. Many printer drivers have color over-saturation (sometimes called "photo enhancement" etc) turned on by default. You probably don't want this. Try turning it off.
In addition, greys are usually built up with colors instead of just black and if you have an absorbent paper (like regular office paper) the different colors will spread and be absorbed at different rates, so the color balance will drift resulting in color casts.