Dell 5100CN, 5110CN Developer Replacement

laser printers have a high freq high voltage wire that puts ions on the paper as it passes ,
have you cleaned this wire . it attracts dirt an inks like the paper it treats.
there is a wire cleaning tool in the machine usually.

This may not solve your problem but it did mine when I got the "reseat cartridge" error. I had inadvertently ordered the cartridge for the 5100cn, which looks exactly the same as the 5110cn and actually fits if you order the generic version (the Dell 5100cn cartridge looks like the 5110cn but doesn't quite fit).

So, first make sure that you ordered the 5110cn cartridge. (And if you need two opened 5100cn cartridges, I've got them up on ebay :( !!!!)

This sounds like a typical Developer Drive failure. The Developer Drive for color is different that the black developer drive. This is why black can print fine, but colors are faint or non existant. see below,a recent bullitin about this exact issue from Dell. ( the ROS, is the printhead, and is located behind the 4 developers.

5100cn Printer
Print Quality Issue
1. Problem: - Color images are extremely faint or not produced, but black images are produced without any problem.
Resolution: - Replace Developer drive assembly (H6702) and clean ROS Window/Glass.
If you chose to persue this repair, you can probably find a replacement developer drive by doing either a google search for the part number listed ( H6702 ) or an ebay search for the same. We can also provide you with removal and replacement instructions.

Good Luck