A question recently came up about someone sending tefillin with his child to camp and wanting to write their name on the tefillin. I'm not sure why he didn't want to write it in the boxes or bag but nevertheless the shaila of writing a name on the retzuot.
I haven't managed to find a tevusha that addresses specifically this question.
I know that having colours on the inside of retzuot is not an issue lema'ase (aside from possibly red) but perhaps writing a chol word would be different? Not so much from a chatzitza viewpoint but from a kedusha viewpoint.
I think some people do write names on retzuot. I would have thought to advise if so to make sure it's on the extra bits hanging down and not on the parts that are wrapped on the arm or head.
Can anyone comment on this?