If you are sticking with one shade, you will want a highlight to define the shape for something that isn't flat.
Notice how, if you don't provide a highlight, the shape could be mistaken for something that is two flat surfaces.
Hue-shifting doesn't work with every colour scheme but it works with many of them (base yellow, orange, blue, green, etc.). You can go across the rainbow as long as the colours aren't too completely different. The first one is a standard hue-shifting palette, starting orange, with a yellow highlight, and shades towards red and purple. The second is an exceptionally vibrant one from yellow to purple and blue. You probably don't ever want to jump colours more than that example.
The bottom is just standard colour changing, no hue-shifting, all colours from the same orange colour wheel. Not everyone uses hue-shifting but I like it since it makes sprites look more interesting. I find the third one decidedly boring compared to the first. It is amazing how much difference colour choice can make.
