Running bouldermark I realised that multicolour text and graphics modes had some major problems.
So I revisited the VHDL code for them, found that multi-colour graphics was simply not implemented at all, and multi-colour text mode had some big fat bugs.
A bit of poking around and sorting things out has it much better, although far from perfect. For example, some games seem to be largely working now:
And bouldermark, while still not perfect, has the right colours for the most part, although one of the multi-colour bit combinations is pulling the colour from the wrong place, as can be seen by the bricks being mostly invisible in the last image. But it is progress nonetheless.