I wrap dry 5x6 large round hay bales around the outside - but not the ends.  I always use white because that is what it available. Even so sitting in the sun it must get very hot under that plastic. I have seen some spots on a bale where it looks "cooked" under the plastic.  Not a significant amount of hay lost but reason not to use black in my opinion.

EDIT: please see Mark's post below regarding color of plastic on dry hay wrap.

One thing about wrapping bales I have found is to make darn sure they are dry before wrapping.  Last year was a tough year to make hay in WI - just no good drying weather as we had last week.  A couple of the bales I wrapped were obviously still wet inside when I wrapped them and lets just say the result was not good.

Where the bales were really fully cured inside when I wrapped them the wrapping makes a tremendous difference in quality as they are opened over the winter and early spring.

Overall wrapping is a positive thing - just dont wrap a bale if there is any question at all about whether it was and is dry inside.  jmho.

Jim at Dawn