Steer clear of ivory as a base, it hates most frits, red doesnt seem to like any of the iris colours, neither does yellow.
The reaction is great if you want to create an aged feel
But if I had wanted just dabs of colour on this bead it would have been ruined.
Most frits seem to work well over white and clear, with all of my frits I do test beads on both colours to see what happens, white tends to allow colour to spread and soften while the clear keeps it crisper.
Quite often if you have even the slightest reducing flame it will blacken the frit, I have a pink blend that is a devil to work with.
If you have a frit failure, super heat the bead and see what happens, super heat it and swirl it around and see what happens. If that fails then reduce it and you may get some great metallics.
If you have done all that and it is still the foulest shade of yeurch you have ever seen then completely cover it in a different colour and start again. No glass wasted and a lesson learned