Due to human perception, light levels, monitor calibration, and all sorts of printing variations (humidity, heat, age of fuser, toner changes, etc), not to mention the difference between colors created by light and colors created by print, it is impossible to do perfect color matching from what you see on your screen to what we print. However, by using color profiles with your image editors, you'll get a little closer than you would otherwise. Download these files and use them with your image editor of choice. Each software package is different in how you assign a color profile, so consult your owner's manual for help on how to do that.
Our website will only accept RGB images. CMYK images will be rejected. However, for the closest color matching, we recommend that designers develop in CMYK and then convert their files to RGB before uploading them to our site.