I do understand the adding a layer, set to "multiply" and paintbrush method. It takes a few steps and longer time to do it. Anyway it just a friendly suggestion to add an additional highlight tool function. The snipping tool in Windows have a highlight button so I actually thought that an advanced software like paint.net should have a highlight button.
I use paint.net mainly for annotation of screenshots. I usually do these things: add text, draw lines/arrows, rectangle, highlight and blank out details.