Yes, you have to. Simplistically, the grid pattern is formed by dark warp threads mixed with the 'base' colour weft threads, and vice versa, and then where dark warp and weft threads intersect you get the unmixed dark colour.I don't know whether to paint the brown intersections a different colour...
How ancient? MacKenzie Ancient for example is green, blue, black, white and red....but the theory is that ancient tartans were 3 colour.