Yes, the mill producing the cloth can make a very big difference in terms of color. For instance, Lochcarron's "ancient green" is a lot lighter than House of Edgar's "ancient green." Each mill has their own color pallette that they use. That's why if a customer calls me and wants to order say, a fly plaid, to match his kilt, I'll always ask if he knows what mill produced the cloth for his kilt, so I can have a better chance of matching the tartan.

That's also why I think it's so silly when some kiltmakers refuse to tell their clients where they get their cloth from (I've actually been told by some that it's a "trade secret"). I wouldn't buy a kilt from someone who wouldn't tell me what mill wove the cloth.

Anyway, I have an article on the different colors found in tartan that some may find enlightening.
http://www.albanach.org/colors.html