Thank you ! For the moment it's not possible to use gardient but it's a good idea for future improvement.
Currently there is a trick to make ring shape with mask color :
You create two centered circle, the small one you put for example green color then in shader mask you add the color in mask palette. (Put the small circle in child of big one to manage easily position)
Some examples here : https://deakcor.itch.io/pixelover/devlog/259755/pixelover-05-shapes-and-effects