Can I refill monochrome laser printer cartridge with color powder?

A little black goes a long way. It's difficult to clean out all of the old toner, so the cyan you get is likely to be an uneven darker version of cyan than the refill toner.

As LPChip points out, the color might not be what you expect. But a bigger difference than the cyan from brand to brand is that the "primary" colors of the toner are rarely the exact color needed. The primary colors are mixed to produce the right color. You would get whatever the (adulterated) toner is.

However, there is a bigger issue than color. Not all laser printers operate at the same temperature and the grain size of the toner is different from brand to brand. A refill powder not design for your brand of printer might not produce good results or might not work at all. And it could produce a mess inside your printer.

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.