OK. No idea what I'm doing. I started creating a lesson about coding this afternoon and completely by accident ended up making this in MIT App Inventor. It's a painting app that you can get from here (Android only). I've published to the PlayStore but it might take a while or get rejected.
And then you can share your work on social media with the hashtag #redbluepaint.
No idea what happens then. Maybe nothing.
You can get the source code here, and make your own version.