Use the adjective scarlet to describe things that are a deep, rich red color, like a robin's breast or a wild strawberry.
Some things are such a vivid shade that red doesn't seem to be quite accurate — scarlet is the perfect word to use for them. Blood is scarlet, and so are rubies and ripe cherries and some brilliant red roses. You can also use scarlet as a noun, to talk about the color itself: "My favorite painter uses a lot of scarlet." In the mid-13th century, scarlet simply meant "rich cloth," which might have been the color scarlet or some other color entirely.