When you are talking to a weaver about a special weave Please, Please spend a lot of time sending color swatches and yarn samples back and forth.

If the Tartan you want has very specific colors then it would perhaps be a good idea to get the pantone number for the exact color you want.
If that fails, then find a swatch of fabric in the exact color or a paint color chip that is exactly the same and send that along.

And talk to the weaver. Talk to them a lot.