"...4000 colours..."
Nah, at least HAM8 has much more, in many images you really can't notice the difference to a 24bit one. But the problem is that the pallete in those images is adjusted.
I've allways though that a cooper(to dinamically change the pallete)/processor routine could be written for almost 24bit color. Then maybe a driver for P96 could be written using that, so that AGA amigas could run programs written for RTG. It would have to be a fast thing though, otherwise it'd eat too much CPU.