If you are using Photoshop 5 or newer you have to set your color profiles to display the image properly. Photoshop adjusts the image color to display properly on screen for you, and the settings from the factory are usually wrong. Those settings are in the file menu. To get a rough calibration, print your image in RGB, convert to CMYK, and adjust your color profile to match the print as closely as possible. If you don't have a printer, then adjust the CMYK profile to match what the RGB was and hope for the best. Color correction is a pain. If you are using a Mac, calibrate the ColorSync Control Panel/Pref Pane as well (well first, actually).