There are two teshuvos which discuss hair coloring: Y’D 1, 82, and Y’D 2, 62. The latter is about looking in the mirror, from which R’ Bick asked a kasha on R’ Moshe’s earlier teshuvah.
In neither did I notice that he explicitly mentions the idea of the changing styles affecting the halachah. If I’m correct that it was already common for men to dye their hair (the second one was written in 5720), then the fact that he assers (when done for cosmetic purposes) would indicate that it remains assur because it’s inherently feminine. It’s not such a stretch to say that there might be things which are inherently masculine, and trousers would likely fit the bill.
Either way, ayin shom for an interesting pshat as to why looking in a mirror is considered beged isha.