A tefillin batim maker offered to paint black the backside of the retzuos of Rav Eliashiv’s tefillin. The Rav, whilst agreeing it was a hiddur mitzvah to do so, declined the offer. He explained that if he had both sides of the retzuos of his tefillin painted black, many others would rush to do so, and there is no hiddur mitzvah when this results in others having to spend money. (see: Mishpacha (English edition) 16 February 2011, page 24)  

Every time one puts on tefillin, especially the shel yad, the retzuos are pulled, and over the course of many years they will, as a result, become narrower and narrower. Also the retzuos might start partially tearing across. A question to be asked is whether there is a minimum width for the retzuos and also whether the torn part can be included in such a minimum width? Some poskim rule that since no width requirement is mentioned in the Gemara, a minimum width is only a hidur, and if there are no other tefillin available, one may put them on and even say a berachah over them. (see: Sha’arim Metzuyanim B’halachah on Kitzur Shulchan Aruch 10:26)

In, for example, a school, where numerous pupils put on tefillin, and thus mix ups between the tefillin of different pupils can occur, it is desirable to be able to identify one’s own tefillin. A question can thus arise whether one may write one’s name (and other contact details) on the back of the retzuos. According to the halachah, one may colour the back of the retzuos with any colour except red. (Shulchan Aruch, Orach Chaim 33:3) Maybe one can learn from this, that one may write one’s name on the back of the retzuos in any colour but red.