You never know when set notation is going to pop up. Usually, you'll see it when you learn about solving inequalities, because for some reason saying x < 3 isn't good enough, so instead they'll want you to phrase the answer as the solution set is { x | x is a real number and x < 3 }.
If the skin really looks like the color of ash, a deeper more persistent gray or gray-blue color, this could potentially be due to a side effect of certain medications, with or without interaction from sunlight.
Causes of Human Skin Hyperpigmentation. The color of human skin is due primarily to the production of melanin by specialized cells in the skin called melanocytes. Melanin produced by these cells both absorbs and scatters UV radiation and thus protects skin from the damaging and mutagenic effects of the solar radiation.