Sounds like the V8 has trouble locking to the color subcarrier from Mainbow, iirc the input monitor out on V8 is just a passive loop-through, which would be the same as bypassing the V8 entirely, and the monitor is probably a bit more tolerant to subcarrier frequency deviation.
Does it change if you input an external video source into the Mainbow? As the Mainbow will lock to the external signal subcarrier, which may be a bit more in-spec with what V8 expects.
If it does lock properly (and shows colors), but goes back to black and white when no external signal is present at the Mainbow input, you can try adjusting the IS (Internal Subcarrier Frequency Adjust) parameter on Mainbow until the V8 picks up colors.
Then, if it doesn’t lock to the external signal subcarrier (output is still black and white), you can try adjusting GC (Genlock Coarse) parameter, while leaving the GF (Genlock Fine) to AUTO. And then try to adjust IS again once no external signal is present at the Mainbow input.
Else, if you’re not able to get colors whatever the parameters on Mainbow are set to, as other suggested, a digital device (like AD/DA converters) in between Mainbow and V8 could help, in order to recreate a subcarrier which may be closer to the specs V8 expects.