Tip for new newbies: to make your own flag work save the file as "256 color bitmap".

I actually plan on making a guide for flags and graves using GIMP and mtpaint eventually. When saving into 256-colored bitmap in MS Paint there's a fixed color palette.

To work around that, inside of GIMP you can make a palette yourself, export the flag as png, and then open it in mtpaint, then save it as bmp.