After searching many hours on the net , I had to resign, no simple color picker exist in GWT.
So I had to do it myself for Promailr.
I wanted something simple to use, not too much colors (not as gmail and not a color picker from photoshop).
It's was not so difficult to realise, a simple Grid with label that takes the color I want.
I used "Chroma" from the google market place to pick color I wanted.
Then I decided to share this piece of code on google code, you can see the project here:
So if you want you can use it or see the code.