Depending on the view mode, sidechains are coloured based on whether they are classified as hydrophobic or hydrophilic. In reality things are less straightforward with some amino acids such as Isoluecine and Valine being highly hydrophobic, others such as Arginine and Lysine being highly hydrophilic, while others such as proline and serine being nearly neutral (there seem to be several 'hydrophobicity scales' in use).

I'd suggest that a gradated colour scheme be used, perhaps using orange for Isoleucine and blue for Arginine with intermediate colours for the rest. This would be helpful on the recent coronavirus puzzles: when you're presented with a sheet that looks like all hydrophobics its hard to know how best to orient it.