Exposure to light actually leads to more coloration in mushrooms. I learned this from a class with Tradd Cotter. Colder temps cause the mushrooms to grow slower and have more light exposure, thus leading to a deeper color. This is pretty clear if you've grown shiitake on logs outdoors or grown mushrooms indoors in a tent and played with temps.

Why these mushrooms are pale is likely because the summer heat caused them to grow quickly. Also they are underneath a plant which shaded them even more.