Stains on colored garments do not render a woman niddah. This is true even if the stain is larger than a gris (the size of US dime or Israeli shekel).
Some poskim feel that if a stain is very large, it is no longer considered a stain but is actually a flow, which would render a woman niddah even if found on colored garments. We have heard of this position of treating stains the size of half a dollar bill as a flow. However, this is not a universally accepted position, and unless this is how you were taught, or you were told this by your Rav, you do not have to treat stains of this size as a flow.
Please see our article on Stains for a more detailed explanation of these concepts, and feel free to get back to us with any further questions.
B’hatzlacha!