The mitzva of yibum is an example of the classic rule aseh dokheh lo ta’aseh – that performance of a positive commandment can push aside a negative commandment. Our Gemara discusses other such cases and their sources. One example is that of tzitzit, which, according to the Biblical commandment that is rarely kept in our day-and-age, requires fringes that are colored tekhelet. While a typical beged (article of clothing) discussed in the Torah is made of either wool or linen, according to the Gemara, tekhelet is wool. Thus, we find ourselves in a situation where the mitzva will be fulfilled by attaching wool fringes to a linen garment, a mixture that is ordinarily forbidden by the Torah (see Vayikra 19:19 and 22:11).
The Torah mentions the color tekhelet on many occasions, but it is not really a shade of color; rather it is the dye from which this color is made. Various discussions in the Gemara make it clear that the blue dye of the tekhelet was taken from a living creature called a hilazon. Because of the many Gemarot that describe the hilazon, it is difficult to identify one particular animal that meets all of the criteria, and there are many different opinions regarding its classification. The consensus of most opinions is that the hilazon is the snail “Murex trunculus” that is found on the shores of the Mediterranean Sea in the north of Israel. This creature has a unique liquid dye (that is not the animal’s blood), which, when mixed with other materials, produces the blue tekhelet color described in the Torah. Already during Talmudic times the use of tekhelet became a rarity, and within a short time its true source was forgotten.
While the Torah does not state this clearly, it appears that the dyes of tekhelet, argaman and tola’at shani were all used with woolen cloth specifically, and it is possible that these unique dyes simply could not be absorbed properly in linen threads.