Conventional subtractive colorheads won't deliver a very deep red effect by mixing Y and M. And being subtractive, there is going to be a bit
of residual white light contamination left over ... so certainly risky trying to do this for safelight purposes. But it was common to use a swing-away filter holder beneath the lens which would hold a very deep red tricolor filter like a 29. Or you could just screw a glass one on to the enlarging lens threads.