First of all, congratulations to square87 for this awesome plugin!
Here are some suggestions:
1) It just changes groups color box in contacts list, for now, but i think it's already planned to change the whole skin colors. For example, a "Dark Minimal" skin was in my todo since this plugin was released, I think it won't be necessary anymore if Skin Color does exactly the same! Anyway, "light" and "dark" colors is a quite good idea;
2) About the button: the default one is just a bit too big, but it's not that relevant since a skin pixmap can hard set a new one. The button is put next to "music" button by default (next to Nickname), it would be better to put it down, next to the personal message;
3) What I think would be important about the button, though, is a keyboard shortcut to hide/show it (or we could just change colors in the plugin configuration window and choose whether to add the button on not);
4) About changing the whole skin colors: it should be able to change font colors too, for example: when I choose "black" I automatically want text to be white;
5) When I'm changing colors, the color picker box does not disappear unless I click outside of aMSN or open a menu, there should be a button to close it or it could just disappear when clicking anywhere but inside of it.
Complimenti ancora, square87!