like others have said already, commercial underglazes/glazes are pretty stable at ^6 even though most of them are low-fire. I like using the underglazes because you should be getting more pigment for your money and they seem to work well mixed into or layered with homemade glaze bases - "really red" duncan is a reliable one for example. not all of them work, but most do.
a tip i learned a long time ago from another sculptor - fingernail polish works wonders for the times that you only need a tiny detail of color on a sculpture (example was irises of eyes or drip of blood, etc) instead of buying stains or UG. it's cheap, looks like glaze unless you're super close, and comes in pretty much every color.