Organize Swatches by Color.

[puggsoy]

I was actually working on a palette extraction and organization tool over a year ago but sort of gave up after having too many issues separating colours. However as Lexou said, simply organising a single palette by brightness should be easy.
I'd be glad to try making something once I've settled (still in the process of moving house at present).

Also once I get round to making that pixel art program (and I will) it'll hopefully have these sort of functions as well.