Currently, the default color is grey for skills that can not be leveled up. However, some skills that can be leveled up have a color that may resemble grey.
I think it should be more obvious if a skill can be leveled up.
I propose that it should have a very noticeable border around it when it can be leveled up or use a more universal system where it is always green for every skill on every hero when it can be leveled up.
I currently do not think the color schemes for the choosing skill matters aesthetically since it's so small anyways.