If you run out of Y or M or Both, you would make things worse if you use Cyan. You will actually need negative cyan, which is not possible. The solution is to use an aditional filter Y, M or Red (Y+M).
I have a Durst Color head which had an additional + 30 Red (Y+M) if you ever had a case where you run out of the regular Y or M
As other have previously said, you should never use the three filters together.
Cyan is commonly used in positive to positive printing (cibachrome/ilfochrome).