One problem is that in many colour lasers there is a LOT of toner between the cartridge and the drum.
In my own printer, I discovered that the third party toner I use can cause toner lift-off at high coverage. However, by the time I noticed it, I was already using my 3rd refill, so there are at least 2 cartridges worth of toner in the printer's piping!.
This means your black may stay black for thousands of pages, and then VERY gradually change to cyan.
All in all, not a good idea.
I overcame my problem by telling the printer it's printing on card instead of paper, whenever I print pages with heaps of colour. This works fine, but it does mean I cannot duplex high-coverage pages.