Voler used to have some black sunsleeves. I guess nobody believed they wouldn't be hot, so they only have ugly ones now. I wish I had gotten the matching leggings when they were available. Maybe the white ones would be better in extreme temperatures, but the black ones have worked for me up to 95F. Taking them off didn't help, so the sun gain must be about the same as with bare skin. The advantage of sun sleeves in temperatures like that is that you can put ice in them.
Fortunately I live in a place where temperatures like that are rare.