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.
It is the presence of melanin that gives the skin its color. A lack of melanin or low levels of it can cause your skin to appear pale, which is known as hypopigmentation. However, in case there is a high amount of melanin present in the skin, the skin gets a darker color, which is known as hyperpigmentation.