why is red the main color in the royal arch

Red or scarlet serves as the characteristic color of the Royal Arch as an emblem of fervour and zeal 12. It symbolizes the fidelity attributed to the degree's prototypical candidates within its traditional history 1, while also representing the earnest qualities a candidate must possess to succeed in the search for truth 2.

Blue denotes fidelity, while purple—combining blue with red—signifies constancy in spiritual combats, according to Portal's study of color symbolism cited by Mackey 2. Beyond this symbolic meaning, purple historically serves as an emblem of dignity, exalted office, and regal authority 2.

In terms of regalia, purple, crimson, and light blue are the three colors traditionally associated with the Royal Arch, though dark blue has by long usage been substituted for purple in the manufacture of regalia, as the United Grand Lodge of England notes.