Is the picture something you created in Word with the drawing tools, or is
it an external graphics file? In either case, I'd say the answer is to go
back to whatever created the picture and use its tools.
If it's a drawing object, click it and use the Fill and Line Color tools on
the Drawing toolbar.
If it's an external file, you might be able to use Windows Paint (or a more
capable graphics program if you have one).