I don't know about any change of color. Horses usually do looke different in winter coat to summer coat. It isn't a problem , is it?
We had sunny weather last week, too! We went for a long trail ride to a place I'd never been to (trailered out to it). Mac was a butt about loading into the trailer. I think it is a bit small for him . He won't load at all unless he is in the first position , which means he gets squeezed by the divider (slant load double). But we had some nice long canters and one all out gallop as fast as he could go. Been a while since I rode a horse at a dead gallop!