We learned that tzitzis strings made of wool satisfy the mitzvah for a garment of any material (except linen), while strings made of other materials satisfy only for garments made of that same material (e.g. silk strings for a silk garment).
Question: What if there is a garment made of wool with a silk lining underneath it – can silk strings be used? How about the other way around?
Answer: The rule is that whatever material the outer layer is made of is halachically the official material of the garment.
Question: On a silk garment, can some strings be made of silk and others of wool?
Answer: No, one should not mix materials of the strings.
Question: Can the strings be colored?
Answer: The Shulchan Aruch writes that matching the strings to the color of the garment is actually a beautification of the mitzvah. The Rama, however, says that the minhag is to use only white strings (not including the ‘techeiles’). The Mishnah Berura writes that it is proper to use a white garment for the talis gadol and katan, and one of the reasons is so that even the Shulchan Aruch’s opinion will be satisfied. The other reason is that there is a verse which implies that a white garment is associated with purity. (Note: The black or colored stripes near the edges of the talis do not interfere with the whiteness of the garment. In fact, they are reminiscent of the techeiles strings!)
Question: We know that linen strings may not be used on a woolen garment because of sha’atnez. We learned also that the rabbis forbade the use of linen strings on garments of other materials as a fence to prevent their erroneous use on wool. What about linen strings on a linen garment?
Answer: Originally the rabbis forbade wearing a four-cornered linen garment even more strongly than they forbade tying linen strings on another material, because a person would tie on ‘techeiles’ (which can be made made only from wool) which would be sha’atnez. Even today, when we don’t have the definite ‘techeiles’ they advised not to wear a linen four-cornered garment which would necessitate linen strings. However, it is better to wear a linen garment with linen strings (and certainly a cotton garment with linen strings) than to wear no tzitzis at all!