Yep. Both skin color and sugar content increase with ripeness, so one is a good predictor of the other. That rule only applies among specimens of the same variety, though. Different varieties have differing amounts of anthocyanins (which give the color), and some of the lighter varieties (e.g. Rainier) tend to have high sugar content anyway. Still, within a batch of Rainiers, those with a deeper blush will be sweeter,