Colored discharge seen exiting the body is considered a flow and not staining. Your status depends on the color of what you saw. (Ideally, you would have "transferred" the discharge onto toilet tissue or a white surface for evaluation.) Dark colors in discharge, even if not "pure blood" can make a woman niddah. A dark red, very dark brown, or blackish color would make you niddah.

Please get back to us or a local halachic authority with a more precise description of the color you saw.