Yesterday I've decided that I'm going to unify both color selectors in the next release.
In Aseprite v1.2-beta10 we'll be able to show two or more color modes (e.g. RGB+HSL) at the same time (Shift+clicking color modes):
As this made the color selector more complex, to avoid reimplementing the same thing in the palette editor, I've decided to finally unified them with an edition mode switcher:
So now F4 key will activate/deactivate the palette edition mode.
As a side note, the old palette editor contained a nice feature: relative sliders to modify +/- hue/saturation values, this will be available through the new Edit > Adjust Hue/Saturation option in a more intuitive way: