I wear a martial-arts related shirt under my gi jacket. I've accumulated some Aikido-related shirts (big disappointment: No t-shirts for sale at summer camp when I was there.
), the rest are related to the JKD universe --either Sifu Kevin Seaman, his students, instructors, or instructors' instructors.
One week in September when it was kind of hot and humid, I was all set to wear a long-sleeved Inosanto Academy t-shirt I'd bought in Queens back in March, but it was so hot I thought I would die with that under my gi jacket. So I switched to the only light white MA t-shirt I have ..... which is also from the Inosanto Academy. (I have a white shirt from Harvey Konigsberg Sensei's dojo, but it feels a little heavier than the other one.) I make a point of wearing an Aikido shirt to local seminars, though.