'Scarlet' originally named a fine cloth, not a colour — the red dye shifted the meaning.
A brilliant red colour with a slightly orange tinge.
From Old French 'escarlate,' from Medieval Latin 'scarlata,' probably from Arabic 'siqillāt' (fine cloth), itself perhaps from Persian 'saqalāt' (rich cloth, brocade). The word originally named a type of expensive cloth, not a colour — scarlet cloth could be any colour, though the finest was typically dyed bright red with kermes. The shift from cloth-name to colour-name occurred in English during the fourteenth century, one of several instances where a fabric's typical colour became the word for the colour itself. Key roots: saqalāt (Persian: "rich cloth, brocade"), siqillāt (Arabic: "fine cloth").
The word 'scarlet' originally had nothing to do with colour. It named a type of expensive, high-quality cloth that could be any colour — blue scarlet, green scarlet, white scarlet were all attested. Because the most prestigious and expensive version was dyed bright red with kermes, the cloth name gradually became a colour name. This shift from material to colour also happened with 'purple' (originally a specific dye, not a colour) and 'orange' (originally a fruit, not a colour).