Is it okay to mix lead-free paints with lead-containing paints?
Usually the lead-free colors are used for the safety of the painter. Adding lead-containing colors to the lead-free colors negates this benefit. Also, the colors are formulated differently, may have different firing temperatures and the lead-free paints have a different flux system. The lead-free flux system may react in an unexpected way with lead/cadmium bearing fluxes.
That said, PELI Glass has successfully mixed lead-free paints with lead-containing paints to achieve specific colors when the paints have an overlapping firing temperature.
Please be aware that the manufacturers consider these results to be experimental and not recommended. So test first!