Thank you for your advice, but so far I can't make it work.
I installed Picasso96 with uaegfx, selected a 68030 cpu and gave video memory to RTG, selected it.
But no resolution appears in the picasso96 mode (I learnt the trick about the settings button, so odd !).
With the checkboard, I get a "no board found.Start your monitor first".
Have you any idea about that ? Do I have to start something or is there something wrong with my installation ?
By the way, after upgrading the resolution to 640 * 512 in 256 colors, you're right, it's more colorful.