The A1200 and A4000 have 24-bit graphics. If the ScanDoubler is only 12-bit then it can only approximate the true colours (24-bit) of the AGA palette. Normal Workbench colours are 12-bit values by default so that they look the same on OCS/ECS, so unless you try and tweak them I'm guessing things will appear normal. Test it with a 24-bit image or AGA coppershade or something. The game Fire & Ice CD32 version would be a good test if you have that to hand.
