Some time ago researchers from University of California published the code for a "neural network" solution, that would automate colorization of B/W images. Judging by the examples and reviews with better than average results.
The code is free for all to use, and if I were a programmer I'd have had a go at making a plugin for Paint.net. As it is I am only an experienced computer user, but that's all.
You can find the more info and the code at http://richzhang.github.io/colorization/.
Perhaps there is someone on this forum that has the savvy to fix it.
