Use of the selection tool (rectangular or elliptical) to define an area, then applying a colour inside the selected area with the fill tool is a standard technique across various raster graphics applications. It's also very simple as a concept and a task to do.
If someone has never used this type of application before, or has spend a long time with vector graphics applications like Inkscape, then there will of course be a learning curve but it's hardly a steep one.
I've lost count of the number of computer applications I've tried where I've said, "How the heck am I supposed to do XYZ?" and after a short time I realised it was quite simple.
The GIMP UI is 'old fashioned' and has some irritations and annoyances but these are in the details. The basics are superb.