URL:

http://a.tile.stamen.com/watercolor/{zoom}/{x}/{y}.jpg

As they explain at the bottom of the homepage http://maps.stamen.com/#watercolor/10/37.4171/-121.6599

"Many applications and libraries understand the notion of map URL templates. These are ours:

http://tile.stamen.com/watercolor/{z}/{x}/{y}.jpg"

So I tried this with "Custom provider" : http:// tile.stamen.com/watercolor/{z}/{x}/{y}&z={zoom}
(I added a space between "http://" and "tile"  so you can see the full url of course not in RWT.
But it doesn't work, I get no texture.

Any idea ?
Best regards, Phil