What color to use for a fill layer? Trial & error! In the case of this icon I chose a brown tone hoping it would enhance the skin tones to a more natural color. I had no idea it would totally transform the image the way that it did! I will say I tend to lean toward brown tones, which add a lot of color to my icons.
Unsharp mask & sharpen actually create different results. Unsharp mask adds a lot of clarity and depth to an icon whereas sharpen just sharpens the edges of the image. I hope that makes sense!
Here's another tutorial I did a while ago which may help you see what unsharp mask & sharpen do: