Thanks for the reply, Enug!
Once the color panel is undocked, there is no problem with changing the mode using the panel menu, as long as the panel stays undocked. If I dock it again, though - the color wheel reverts back to square.

On another note: I noticed that you've got a three-monitor configuration. I am curious how does AR behave in your configuration?
I have this problem that contextual menus in AR appear not on the monitor I'm using in AR. I have a mac laptop + Cintiq + monitor configuration that works OK in other graphic apps ( like Photoshop, Krita etc ). I use AR on my Cintiq's screen, but all the contextual menus ( the ones I have in various panels ) open on my external monitor. That is, I click on a button on my Cintiq - and a menu opens on my monitor. That's quite annoying, plus the place where the menus open on my monitor's screen is mostly random, so finding them takes a while, as I have a fairly large monitor... I have described this in detail here.
As you also have a multi-monitor configuration, I'm curious whether you've had any experiences with AR such as mine. I hope not, as this is a quite annoying behaviour...

Mikołaj