2,062. How to Compare Colors

Hilchos Issurei Biah 5:11

One compares the two shades as follows: he takes the part of the cloth with the stain in his hand and looks at it and at the ink, the saffron, the blood in the cup, the muddy water or the diluted wine in the cup. He evaluates them according to what he sees and rules the woman either unclean or clean. He doesn’t look at the cup from the outside; rather, he looks at the liquid in the cup. The cup should be wide, it should weigh a maneh (about 14 ounces) and it should hold two log (about 26.5 fluid ounces). This will enable light to enter it and not be obscured by shadows.

Hilchos Issurei Biah 5:12

A stain is only checked on a white cloth, in the sunlight. The one examining it uses his hand to cast a shadow over the stain while standing in the sun. This enables him to see the stain as it truly is. It’s not necessary for everyone checking stains to do all this every time. Rather, experts develop a keen eye so that when they see stains, they immediately know whether to rule it unclean or clean. Only in a case of doubt must he bring it close and compare it to ink, blood or the other aforementioned colors.