I'll second the Wizard's advice. From painful personal experience I know that you can never be too specific.

A few years back I wants to recreate a custom weave I ordered some time previously. Although I ordered from the same weaver that did the initial run, and provided them with a sample of the tartan that I wanted to reproduce, the colors in the fabric that I received were not even close to the original.

It seems that the weaver in question had changed yarn suppliers and the colors were the "best match" they could manage to the original using yarn on hand.

Now, if I had been aware of that at the outset, I would gladly have paid extra to have the yarn custom dyed to match the original hues, but as it was, I ended up with several yards of cloth that were unsuitable for the purpose for which I intended them.