Wire haired terriers know how it works. They wear wiry hair through summer to shade their skin while allowing cooling air circulation. Their softer and fuzzier undercoat that develops in autumn provides better insulation from cold weather through winter.
Beards are not naturally so adaptable, but can be manipulated to function better. Thick beards might insulate and deflect wind through winter. Short beards may not work as well in that regard, but might absorb warmth from sunlight if still darkly colored. As weather gets warmer in spring, beards can simply be removed to optimize cooling air circulation, and eliminate the extra insulation. However, long and gray beards can provide shade without absorbing much warmth from sunlight.
Have you ever noticed that plants that live near foggy coasts or in the lower levels of shady jungles and forests are the darkest green? They want to absorb all the sunlight they can get. Monterey pine, Monterey cypress, coast redwood and the many ferns that live on the ground below them are classic examples.
Plants that live in exposed situations or high elevations are lighter green. Some have glaucous foliage that is shaded gray or blue to reflect a bit of sunlight. Colorado blue spruce, silver leaved mountain gum, century plant and silver sage exemplify this technique. The old man cactus actually shades itself with weirdly elongated spines that resemble gray hair.
English walnut, maple, apple and pear have sensitive bark that would get burned if too exposed during summer. Yet, they are all deciduous and bare through winter. They do not mind, because the intensity of the sunlight through winter is inhibited by the low angle of the sun. By the time the sun is higher in the sky, and the sunlight is more direct (passing through less of the atmosphere at the higher angle), the sensitive bark will be shaded by new foliage.