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.
Melanin is the substance that gives skin its color, or pigment. When the skin cells that produce melanin (melanocytes) reduce their production, the resulting condition is known as hypopigmentation (loss of skin color). Hypopigmentation is also known as skin depigmentation. The most common cause of hypopigmentation is damage or trauma to the skin.