Boyd’s Bible Dictionary:

(orange-red). A Tyrian color much prized by ancients (Ex. 25:4; Prov. 31:21).

Concise Bible Dictionary:

The word most frequently translated “scarlet” is shani, and this is often accompanied by the word tolaath, “worm or grub,” apparently intimating that the color was obtained from some insect, as it is now from the cochineal.
Cochineal bugs on a prickly pear plant.
Scarlet was much used in the needlework and hangings of the tabernacle, in conjunction with blue and purple; but there it apparently refers to some fabric of the color of scarlet. If the purple be taken as symbolical of royalty and universal dominion, the scarlet may signify earthly grandeur and Israelitish royalty (Ex. 39:1-29; Josh. 2:18,21; 2 Sam. 1:24; Prov. 31:21; Song of Sol. 4:3; Isa. 1:18). In the New Testament they clothed the Lord in a scarlet robe, κὀκκινος (Matt. 27:28) it is “purple” in Mark and John: it may have been an old faded robe that could be called either. Scarlet is also employed with purple to point out the earthly grandeur of Papal Rome (Rev. 17:3-4; Rev. 18:12,16).