Mazal tov on your new baby!

Not all colors put a woman in niddah. This is the case with some browns. If the stain is a light brown (the color of coffee with milk) with no reddish hue, it is acceptable and you may continue your count.

If the stain is darker or appears reddish, you should continue counting your clean days and drop off your underwear when you have an opportunity. You should take care to continue with your bedikot so that you have a valid hefsek and day one bedikah if the stain is a niddah color.

Many women understandably feel uncomfortable bringing personal items for evaluation to rabbi. If you are willing to sacrifice the underwear, we suggest cutting out the area around the stain, and sending in just the piece of fabric with the stain on it, which may be less embarrassing. Just be sure to write down that it is a stain is on clothing and not a bedikah cloth. You might prefer to send your husband to drop it off for you. There may also be a yoetzet halacha in your area who can assist you; a list of yoatzot worldwide is available here.