Defining white & colored underwear

A stain without hargashah on a colored surface does not render a woman niddah.  So we generally advise wearing colored undergarments when a woman is not in niddah. 

White undergarments are required during the seven clean days.  If a woman does not have a clean pair of white underwear, she can wear stain-free underpants of a different color -preferably light– until the white becomes available.  The following is a discussion of best practice, to keep in mind when purchasing or laundering underwear in preparation of the clean days or choosing underwear on the clean days.

1) Underpants completely lined with white would have the status of a white undergarment.  The lining in this case would be considered the undergarment and the colored layer an additional layer.  Such an undergarment would be suitable for the clean days.

Turning such an undergarment inside out for when you are tehorah is not a good idea, since a stain could easily soak through to the white.  (This goes for some cases below as well).

2) These are not useful from a halachic perspective.  They would not be adequate for the clean days, unless very tight fitting, since underpants can shift around and blood might fall on a colored area and go unnoticed.  But a stain on the white area would be treated as a stain on a white garment.  Thus, these underpants would not help you avoid stains when you are not in niddah.

3) Since the crotch area is colored, this undergarment would not be suitable for the clean days.  Although the colored area would be helpful with regard to stains when you are tehorah, you would be taking the risk that a spot on the undergarment if it should shift, could land on the white part (where it could render you niddah).  These might be most appropriate for times when you are confident that you will not stain.

4) If the lining is separate and could be pulled apart from the outside, it would be no different than 1. 

If the lining cannot be pulled apart, it would not be appropriate for the clean days, since it does not appear white and it would be impossible to accurately evaluate a stain's color.  Since there is a white layer, it also would not be considered a colored surface for stains. 

If it is not a separate lining, just the inside of a colored garment, how to evaluate it would depend on its exact shade.  If the inside is white or off white with no hint of other tint, you could treat it as a white undergarment and use it on the clean days (and not wear it when you're tehorah).  If it were tinted another color, you would treat it as a colored undergarment.  Note, however, that some authorities hesitate to treat pastels as colors when it comes to evaluating stains.