To colour your own image in you should try this. When you are done
drawing you image pick a bright color like hot pink. There is a little
paint bucket on the side. You click that then click the background of
your picture. Is should color the whole image the color you have
chosen. If your drawing is colored the color of the Background then
that means there is an opening. You have to make sure all the drawing
is connected. Then if it's connected then you should be able to color
it the color you want.

Well...if you have the hand drawn picture already saved(JPEG, GIF,
PNG)then all you would have to do is open the picture and then paint
it.

... MS Paint isn't the best program for coloring pictures. If I were
you, I would go with OpenCanvas or Photoshop, perhaps a free trial.

But for MS Paint, I think you can upload your image onto it,
and then ink and color over it.

On the MS paint program there a paint bucket. Click it then you click
inside your drawing and its colored.