It should work like Photoshop does in the U Tube link. You can use various methods to select each area to colourise. I do have Photoshop, and not Paint.net, but I am pretty sure it would work. Mind you a freebie is unlikely to give as good a result as Photoshop that can cost anything from £80 to £600 ! See
Have also come across this . I've had a play with it. Open a similar coloured photo on the right side. This is just to get the right colours for different sections. The monochrome photo is opened to the left. It's an online tool and you save the picture that opens in a new tab. Right clicking the finished photo and you can then 'save as'. Worth a look anyway.