Un tapis en velours violet.

would be the most usual word order. The colour refers either to the rug or to the material.

If I said :

Un tapis violet en velours.

I expect it would not shock anyone. The colour only refers to the rug.

But you’ll have to be careful with a feminine or plural noun because of the agreement.

• Une moquette en velours violet.

  The colour definitely applies to the material (velours is maculine)

• Une moquette en velours violette.
  Une moquette violette en velours.

  The colour applies to the carpet (moquette is feminine).

A search with chemise en coton bleue and chemise bleue en coton shows more hits for the material being placed first, but the difference is not very large.*

Chemise en coton bleu (where the adjective qualifies the material) returns a lot more hits but this figure is not reliable because of all the occurrences where bleu is modified (bleu clair, bleu foncé, etc.) and therefore invariable. We do not know if the colour describes the item or the material.

^{*I have tried the search on something more common than a purple rug…}