Probably not the adapter. They are just relocating the wires to another pinout after all. I've never had one go bad fwiw.
First check for a loose connection anywhere in the cable. I've seen that cause this kind of problem.
Try a another cable or adapter.
Next , enter the bios and if those colors are ok then boot to windows ans check the color settings in Windows. Perhaps someone messed with them.
Heres a guide to check/change your color calibration:
http://windows.microsoft.com/en-US/windows7/Calibrate-your-display
If the bios colors are also off see if you have color adjustment control on your monitor or its menu's
Next step would be to make sure either the 550ti or the monitor is definitely working ok. Either try a known good monitor with the pc (tv's often have a vga input) or connect your monitor to something else you know works good right (got an old laptop with a vga output?)