Does anyone know how to add color to a piece of art (paint, ink, art)
One option is to scan your line drawing into a image file for your computer, then use that to experiment with colors on your computer screen where mistakes and poor results can be undone with a mouse click.

Once you find a set of colors which work for your particular drawing, you can either print it out on a color printer, or if you are looking for more authentic, human results, you can brush in the chosen colors on the original canvas, turning your line drawing into a painting.

There are several computer programs out which allow you to mimic different brush and paint types on the screen.  I use the free software that came with a Wacom tablet which I bought a bunch of years ago, but I have seen videos for Photoshop and CorelDRAW brushes which look a lot more capable than what I use.