Turns out he is much more traditional than I thought!
Never in a million years did I think he'd care what color dress I plan to wear! I mentioned a dress that I liked and said I was considering buying it in black. For whatever reason, he thought I was talking about wedding dresses, so I knew I had to run with it. We tease and mess with each other all the time, and I never had any intention of getting a black dress anyway. I stopped when I realized he was genuinely upset, but he went as far as to beg his sister not to let me get a black dress when I brought her shopping. (She told him he was being an idiot and that I was having fun messing with him because he got so worked up over it.)
Based on the above, I should have known he'd care about bridesmaid dresses, but when it came out that I told them to buy a dress in a dark red within their budget and wear whatever shoes they felt comfortable in, he started lamenting about how pictures would look weird if the dresses weren't all the same exact color. Not my hill to die on, and the wedding party wanted more direction so I said ok, I'll pick color and designer if it will make things easier for you girls. Funny thing is, FI is colorblind.
Anyone else going through something similar? I'd probably feel badly about how much amusement this brings me, but he always pays me back and completely asks for it!