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.
The Gray gene causes progressive depigmentation of the hair, often resulting in a coat color that is almost completely white by the age of 6-8 years. Horses that inherit progressive Gray can be born any color, then begin gradually to show white hairs mixed with the colored throughout the body.