Look closely at a plaid crossing and the palette begins to multiply.
Navy, Dark Green, Firebrick, Goldenrod, Wheat, and Black run in both the warp and the weft.
A matching pair produces a solid square; two different yarns alternate in the twill and are blended by the eye.
The extra shade exists in the weave, not in a seventh dye bath.
That simple mechanism is why six thread colors can make a tartan look far more crowded with color than its yarn inventory suggests.
