I do not know if you have tried the Coloumbia Sportswear co. Team High Road is now sponsored by them. (How lucky are they?) I have used their "fishing" shirts as over shirts for hiking and generally being outdoors in the summer. For hiking, my "undershirt" is something like a good wicking tee. Now that they are making them, I might check into a mountain bike jersey. They are lightweight. There are light colors and they have vents in the back.

I think LLBean sells something similar in their travel wear sections.

I hope this helps.

Ellen