www.getpaint.net/
and
www.gimp.org/
those are the sites to get Paint.net and GIMP take a lok at the info and see if its something your interested in, if your looking for a program.
sai.detstwo.com/sai/
and painttool SAI is free for 30 days only costs i'm told $60 for liscense. after your 30 days are up. IF your looking to get more art-ish programs at a NO- low cost.
As for Microsoft Paint, while its not impossibly to use it will be a bit more on the painstaking said to attempt to color in this program in great detail. most lineart when broken down is not "clean" to the point that paint in MSpaint would be easy. Not to say it cant be done b/c it most certainly can be done, BUT it's not exactly easy. And when saving the files tend to be MASSIVE on your memory space. Or at least this was my experiance way back when.
The 'art form' Oekakai, is usually very similar to using MSpaint. ^_^