Greenshot Image Editor should have a a color palette to allow you to change color in one click.
Currently in Greenshot v1.0 to change color you have to do 3 clicks:
1) Click on either Fill or Line color button.
2) Select the color.
3) Click on Apply
In mspaint on Win7 the line color can be changed in one click. In my workflow I almost never use filling, however I often change Line Color (between red and green mainly). I wish you improve this in greenshot.
Thanks for your great work on Greenshot, it's awesome!