I think stability and size matters for this. Whenever I used such in the past the shortcut was either too hard to quickly press or it popped up too big or too small for me to see the underlying colours or to make an accurate selection. I tend to work with quite narrow values so I need to be ultra-precise with value steps. An indicator large enough to compare the previous colour also helps because when you are painting and a big colour wheel is now in the middle of the screen covering your painting it can be hard to see the value changes. I think it would be better for the comparison to be made with a series of the recent colours as you can't always guarantee picking the right one immediately. So the colour wheel should at least have the option of scaling and recent colours that are large enough to distinguish against a preview swatch of the current colour.
In fact tbh, instead of just popping up the wheel in the middle or at the cursor, just having a colour wheel that is say more on your drawing hand's side (so movable) would be easier because often the colour picker is either too far away or covers up too much of your focus area.
I'm not sure if this should work on release of the key or the pen though because moving the hue and sat/value are two distinct steps but in the case of swatches without a colour wheel often you just want to click once. Mind, I never use swatches outside of the history ones sometimes I just feel other users who do would benefit from that. As Carles said, mixing palette is also a good idea though I tend to just outright pick colours myself, I can imagine it'd be very useful especially for beginners and people used to traditional painting. For me personally, access to the colour wheel is more important as I waste a lot of time reaching for a colour wheel that often I have to size smaller than necessary because it just takes up too much of my screen when I'm not using it.
