As stated by others, it sounds like the monitor itself. Power and heat issues with video cards usually result in dropped signal or sluggish performance rather than color issues.
Also, it may seem simple, but you might want to check your connections and cables. If you have a bent pin on a cable, every card you install in your computer will react the same way- sometimes working when tilting (possibly completing the circut?). I had a similar issue in the past (my screen was shaded white though), and it was due to a half-seated VGA connector. It looked flush, but a little push brought it a couple of millimeters closer, and fixed the problem. If you have DVI/VGA heads with thumb screws, use them. Futher, make sure the graphics cards are firmly seated in the AGP/PCI slot. This is another area where they can look flush to the eye, yet the slightest deviation can create strange visual effects.