My color cartridge indicates full - but today the colors print very faint like it's run out of ink.

Yes the colors are mixed with the black for different shades of black when printing.

You risk you ruined the cartridge with alcool.

It sounds like you have a severly clogged print head. The nozzles are likely gummed up and need to be cleaned. To do this, you need to inserta new set of OEM cartridges. Run the deep cleaning utiliity a half dozen times and then the alignment utility. This should clear the problem.
You have most likely been using aftermarket/reloaded cartridges with inks that do not meet the manufacturer's specifications. I would take them back to the vendor and ask for either a refund of replacements. If you are going to continue to use these cartridges, I would take out the OEM cartridges, reseal the two openings on the cartridge ( use electrician's tape) so that they can be used to clear the nozzles should they again become clogged with aftermarket ink.
Let me know if this solves your problem.

Indicates that the printer is using only color ink to create a black color. Check printer settings to see if you can set it to use only black cartridge. Also try printing with only the black cartridge inside if the printer permits, this way you can be sure if the black print head is faulty.

Take the cartridge out and clean the contacts off with something (ie: pencil eraser, your thumb...) That worked for me! I couldn't find a solution anywhere even after chatting with online tech support from HP. Quick and painless! I hope this helps...

Did you try to wipe the cartridge.Maybe there's a clogs. inside.

Mostly it connot print w/out color ink if when printer message empty / sometimes the printer no message while ink was empty still black can print. it depends to the chip .

I used some less expensive generic ink cartridges on my RX500 and had a similar experience with the black. If you're already using the Epson brand, done the head cleaning, realignment and nozzel checks, I'm not much help. Try Epson's support at their web site.

How may times did you use the printer to print befor this happened? If you used it a lot then you may need to buy more ink. My printer showed full ink and it turned out that they were empty.