Reusable colored cloth pads do not make a difference in your niddah status during your menstrual period. A flow of blood makes you niddah even if you find it only on a colored surface. Furthermore, you may already be in niddah when you start using the pads, if there was already a flow or if you had staining, which can sometimes cause niddah.

Colored cloth pads might make a difference if you wore them to absorb light staining (as opposed to a flow). Such pads are halachically equivalent to colored underwear, and a stain found on colored underwear does not make you niddah. Please see our article on Stains for more details.