Is it permitted to wear something new on Rosh Chodesh Iyyar?
The Shulchan Aruch (493) rules that one does not get married or have a haircut during the Sefira. There is no mention of not saying Shehecheyanu, and indeed the Ma’amar Mordechai (quoted in the Mishnah Berurah) permits it.
Nonetheless, there are sources for the custom that many have not to say Shehecheyanu during the Sefira, and some poskim even extend this to not wearing new clothes during the Sefira (even according to the prevalent custom nowadays not to make a Shehecheyanu on new clothes). For those who follow this custom, some authorities permit new clothes on Shabbos, but there does not seem to be a source for permitting this on Rosh Chodesh.