This excerpt from "Scottish Clan & Family Encyclopedia" by George Way and Romilly Squire. There is a section on tartans in the book, by Alastair Campbell of Airds, which may help. These three Gentlemen are probably amongst the foremost experts on the subjects of Clans, tartans and kilts.

Page 40:

".....there is no strict law which lays down the shade of colour to be employed, unless a sett specifically contains a lighter and darker form of the same colour. Campbell of Cawdor, for instance, contains both light and dark blue but one manufacturer's "light blue" may in fact be darker than another's "dark blue". Red is red and green is green, whatever kind of red or green it is and many variations may be found; for this reason, the modern practice found in some cases of specifying such niceties as "plum" or "cerise"( my words;- "white or off white") is to be deplored; there are too many possibilities of interpretation for such specifics to be clear and the Lord Lyon will dissuade would-be users of such terms...."

Interestingly for future tartan designers to also bare in mind, he goes on to say:-

"....another entirely modern conceit for which there is no historical foundation is the claim for symbolism in the colours employed-gold for the cornfields, green for the pinewoods, blue for shining rivers, and so on. In some cases this has been carried as using threads of various colours in numbers which reflect a date or some significant number, a fantasy which is unlikely to lead to good design."