The Mishna (2a) described that mar’ot nega’im – shades of leprous marks – are “two that are four.” That is to say that the two nega’im – the two signs of plagues of leprosy mentioned in the Torah (see Vayikra 13:1-2) – a se’et or a baheret – each have toladot (secondary marks) – other, lower level signs of this plague – that are similar to them in color. Our Gemara quotes a Mishna in Massekhet Nega’im that teaches that a se’et is the color of the white wool of a newly born lamb, and its tolada is the color of the membrane of an egg. A beheret is the intense white color of snow and its tolada is white like the lime plaster of the Temple sanctuary walls.
Rabbi Ḥanina notes that this Mishna disagrees with Rabbi Akiva who rules that the different shades of white are hierarchical which will not allow a nega that is the color of lime to be joined with another nega to be the appropriate size.
There are two main explanations in the rishonim regarding this disagreement.
According to Rashi, the author of our Mishna and the Mishna in Nega’im believes that there are two categories – se’et and baheret – each of which has a related nega one step lower that the main nega. The nega’im in each category can be joined to establish a full nega tzara’at. Rabbi Akiva, on the other hand, does not see a number of categories, but one list of nega’im, and each one can only be joined to a nega that is right next to it on the hierarchy.
The Ramban and others view the disagreement differently. They believe that even Rabbi Akiva agrees that the different nega’im can be joined to each other. They argue that the difference is one of definition. The Sages of the Mishna believe that the two categories of se’et and baheret are not simply two different levels of brightness of the same color, but are essentially different hues of white, and each of those shades has a second level nega that is similar to it. According to Rabbi Akiva, however, there is only one hue of white and the four nega’im are different levels of brightness.