Cyan Toner always prints faded.

It is the developer. All colours should be changed at the same time. This can be accessed via the right hand side of the machine, when you remove the cover. (where the output tray is)

One interesting thing about light copy on a Doc 12 is, the toner cylinder has a wire spring inside it. The machine turns this spring (called toner stir) in order to shovel the toner out the hole so the toner can go into the auger to transport the toner to the developer housing. If the toner stir motor(s) aren't turning, hardly any ink will go into the auger. The ink just sits in the toner bottle, not moving.
Eventually, the problem color will get light, making it look like weak developer. Tap the toner cylinder several times, run some copies, eventually the problem color will start to get darker.
If a test pattern is not available, then print a "small" image of a copier test pattern. A large test pattern will use a lot of toner.
The solution is, have the toner stir motor circuit checked. Dirty ADC sensor, laser mirrors dirty can block light causing poor copies.. The toner augers can also be faulty.(Broken auger, or toner motor gears not rotating.)
As a suggestion earlier, one pulling out the developer housing, adding a teaspoon of toner in the developer housing will help prevent the machine going into an error codem because of the light condition. Put the toner in and light shake the developer housing will help stir the developer-toner mix.
Some copies would have to be run in order for the developer housing to stir up the developer and mix the added ink. Hope this works.

Its a dc12 (xerox docucolor 12)

What kind of printer is it? Also have you replaed the drum cartridge? Are they both in the same unit? If so replace the whole cartridge.