So, I've now done more tests on eight different sorts of clay. Horizontally each line is one sort of clay. Vertically the left column is fired to ca 1140ºc, the central column in gas reduction to ca 1270ºc and the right column in electric oxidation to ca 1270ºc.
As expected, the grogged porcelain sample at the top is the whitest. I am astounded how dark some clays go when fired in reduction.
The red and blue underglazes keep their colours quite well even in the higher temperatures and seem to be more affected by the change of clay colour than temperatures.