Could be one or more of many issues. Most likely you are simply running low on ink. Does your printer have a single color cartridge and a black cartridge? Or does it have 3 or 4 different cartridges for colors, and a black cartridge? The multi-cartridge color printers are more efficient, because when Yellow runs out, for example, you spend less money to replace just the Yellow cartridge, whereas the single cartridge for all colors is lame--if you use more Yellow than usual, it will print crappier when the yellow runs low and will tell you to replace the cartridge when there is plenty of the other colors inside the cartridge.
The other main problem, as someone mentioned, could be the drivers. Try reinstalling them with the original CD (hopefully you still have it). If you need to, you should be able to download the drivers from the printer manufacturer's website. Just find your printer model, enter the model number in the website's search tool, and go to a link for the downloads and/or resources.
Hope this helps!
Over 10 years experience working in print shops.