Ketamim only make a woman niddah if she found the ketem on skin or white clothing. If she found a ketem on colored clothing, she is not niddah. Light colors are considered colored and a ketem found on a light colored garment will not make a woman niddah. If a ketem is found on a garment which is very light, such as off white, a rabbi should be consulted.
If the clothing was spotted, and the ketem was partially on white and partially on colored, she is niddah only if there was a gris of blood on the white part of the clothing. It is a good idea for a woman to wear colored undergarments and use colored bed sheets, as this will save her from potential ketamim. It is important to note that during shivah neki’im a woman must wear white undergarments.