From OED, second edition (1989):
scarlet, noun and adjective
2. a. A brilliant vivid red colour, inclining to orange.
Edit, a few more OED entries for comparison:
red, noun and adjective
A. adj. I. 1. a. Having, or characterized by, the colour which appears at the lower or least refracted end of the visible spectrum, and is familiar in nature as that of blood, fire, various flowers (as the poppy and rose) and ripe fruits (whence the frequent similes red as blood, fire, a rose, cherry, etc.).
The precise shades of colour to which the name of red is applied vary from bright scarlet or crimson to reddish yellow or brown (the latter esp. of the hair of certain animals). The numerous varieties are distinguished, when necessary, by prefixed nouns or adjectives, as blood-, brick-, cherry-, fire-, flame-, flesh-, robin-, rose-red; dark, dull, light, lively red; fiery, foxy red; brown-, orange-, yellow-red; brownish, yellowish red, etc. For examples of these, see the first element.
3. c. Prefixed, as a qualification, to adjectives of colour: Deep in shade, absorbing more light than it reflects; the opposite of light. (Usually hyphened with the adj. when the latter is used attributively.)
crimson, noun and adjective
A. 1.The name of a colour: of a deep red somewhat inclining towards purple.