Hi, I attempted to contact you on Discord but this extension seems to have a bug when used in Aseprite v1.3-rc6. In Color Groups, when adding colors, the color at the last index in a palette can't be added. Instead, the color at index 0 is added. This seems to happen in every palette I've looked at.
Edit: found the issue. In color-groups-widget.lua you have
if index < ncolors - 1
but Lua indexes start at 1, not 0. Changing it to just
if index < ncolors
then fixed the problem.