I've done a bit of riding in SE Asia and for any distance work I settled on a Levi jacket, t-shirt/vest and jeans with fingerless cycling gloves -plus a good open face helmet and sturdy walking boots. In say, a Bangkok traffic jam, I can just about put up with the heat wearing that lot, anything more would seriously put me in danger of heat exhaustion I reckon and there have been occasions where the jacket has had to come off and I rode in just the t-shirt.
The trick is to wear loose but resonably protective clothing, it's essential that your body is able to lose heat.