To expand on Matt's point, in The Setts Of The Scottish Tartans DC Stewart says:

...the majority of tartans fall into one of two almost equal groups- those with a green ground and those with a red.

A considerable proportion of the designs of the green tartans have a common structural basis, consisting of a broad blue band contained within narrower bands of black...

On this foundation is built up a scheme of black, red, yellow, light blue, or white lines. Black lines, only, are employed in the Black Watch tartan. In the others it is usual to find light lines in the green rather than in the blue band; where two bright colours are used, and distributed over the green and the blue, the lighter is usually in the green.