I'd love a tool which can be used as a simple color checker anywhere on the screen.

Perhaps as a separate tool it would be a simple disc which has the same color as the current color... it would be located at the cursor and could be moved over the painting or a reference or scraps or views to enable the user to judge the color by eye.

A hotkey (requiring one to hold down similar to the way the color picker tool is activated by the "alt" key) bound to the color checker would be very useful for quickly making the sure the current color is appropriate for whatever I am about to do regardless of what the current tool actually is or even where the cursor is.