Most image editing software I use, you can do exactly as the title says. Left click for your main color, and right click for the fill/other color. It's so standard that I was actually surprised that you have to change the fill in this program by clicking color -> Clicking fill -> Selecting "Assign" -> Clicking another color for the edge you want.
Especially so if I'm playing around with colors and haven't decided what I wanted yet. Can make that process a bit tedious.
It doesn't hurt to ask!