I appreciate all the good responses, guys. Perhaps ASA should put something down in writing for us all to see and abide by. Just one more thing to help keep our uniforms exactly that: uniform! It would also be one less place to potentially get dinged on a ump's eval.
While most tourneys do take place in the summer months in areas that can get cold, even the warmest of environments get bizarre cold spells at the most inopportune moments. I've seen umpires with white gloves, I've seen umpires with black gloves. I've seen umpires with blue sweatshirts under their jackets that are visible, and I've seen umpires with black sweatshirts. Many of these umps even call at the national level, and I look to them for guidance when I call with them. However, in this case, seeing the variance I've seen from even the vets, it's never clear to me that ASA accepts one method over another.
In the end, I throw on my old pair of black batter's gloves, toss a couple of those heat packets into my pockets, and don an extra pair o' socks when the temp dips. Works "darn good" for me.