Ah now it's getting interesting :) So far I could reproduce the issue across several machines, also on new demo machines at the Apple booth of electronic discounters - so I don't think it's something about my configuration, but maybe it has something todo with how I'm using the trackpad (but I'm just sliding around with the right-hand pointer finger).

PS: the mystery might be solved => that buggy 'Custom Colour' UI item only shows up under specific circumstances, which for my specific usage pattern is 'obvious' - see my sister comment for details.