Yes actual swatches are a must, but even when the swatches come from the same mill there sometimes colour differences between the various weights.

You all know about the lovely "muted" range woven by House of Edgar.

In my avatar I'm wearing my band's tartan, Drummond of Perth, in HOE's muted colours.

Some of our band's kilts are 13oz, some 16oz, and the colours are not quite the same. The red in the heavier weight is slightly more purplish.

Oddly, the same tartan is listed as being in their "muted" range in one weight, but in their "modern" range in the other weight, in their sample books.