2,121. Blood or Dye?

Hilchos Issurei Biah 9:35

The rule in the previous halacha (that all three women in a bed are unclean) applies when the women all checked themselves and found that they were clean so none of them could ascribe the stain to another as has already been discussed. However, if one woman checked herself and found that she was clean, then one who is clean can ascribe the stain to the one who didn’t check. The woman who didn’t check is unclean.

Hilchos Issurei Biah 9:36

If a stain is found on a woman’s garment and there is no outside cause to which it might be attributed, it doesn’t render her unclean until it’s determined to be blood. If there’s a doubt as to whether it’s blood or red dye, the experts who examine the stain check it with seven “detergents” in their designated order. If it washes off or the color fades, it’s blood and the woman is unclean. If the stain remains the same, it’s dye and the woman remains clean.