Right now, it's not possible to change the sorting of colors in a palette directly from the UI. I wish it was possible to drag and drop colors freely around the palette grid. Maybe even select multiple colors and then drag+drop them while keeping the same alignment. I'm not sure how to solve the problem of dropping colors in place of other colors, would they get pushed, displaced, switched, overwritten?
Why would you need to rearrange the swatches for? Switching makes sense when you only need to adjust a couple of colors on an already well layout swatches. Inserting dropped elements in place, makes sense if you want to rearrange all to make your own swatches. At the moment I could not find a way to save new palettes, so rearranging by hand makes little sense to me. How about sorting of the palette colors according to the color properties? Ordering the available swatches could be done using Luma strength, Hue value, saturation, lightness, etc. Sorting the colors of the palette makes a lot more sense as you can find color similarities to apply to your work. Of course the "default" sort is the palette default state. I think sorting manually only makes sense when building palettes.
That's right, I suppose I was implicitly asking for the ability to retain and save palettes within Krita, an ability that is currently lacking. You'd wonder why Krita allowed us to add new colors to existing palettes and not keep those throughout sessions. Sorting options would be an acceptable short-term compromise as well.
The ability to rearrange colors is a request I see from Japanese users form time to time. (another request is the ability to add "name" property for each pallet color - such as "skin 1" etc) They hope to arrange colors specifically for their use, not by color properties - so sorting option would be a short-term compromise, but not a solution.
Part of the 2015 accepted stretch goals, subject to the final UX design. WISHGROUP: Stretchgoal
Krita 4.2 has the ability to re-arrange color order in the palette docker. You can also give colors friendly names.