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.
Cyanosis means that there is less oxygen being carried by the bodyâs blood cells. In this situation, there may not be enough oxygen being carried to the tissues in the body. The colour is caused by high levels of reduced (deoxygenated) hemoglobin in the blood near the surface of the skin. Hemoglobin is the molecule that allows red blood cells to carry oxygen. Oxygenated hemoglobin is bright red, while deoxygenated hemoglobin is bluish.