Long story short, I recently prep'd a body with 3220 Mirocat, sealing it and building up a more-or-less level(ish) base that I was planning on painting in full-gloss opaque black:

I dropped into my local Mirotone supplier today to get them to mix me up said black, but was told Mirocat can not be tinted. I was always under the assumption 3220 could be tinted (since Mirolac can), and also because tinted Mirostain can be added to 3220 (up to 10%). After hearing this, I asked whether Mirolac 3150 (which can be tinted) can go over the top of 3220, but was told it can't.
So, it seems I've painted myself into a corner. What are my options?
Thanks