I had a problem changing the color of my avatar in a campaign. When clicking on the color square near to my avatar name no color picker popup appears. And I found a solution / workaround. The problem only ocurres when the avatar is placed in the top position as you can set in the option (gear) tab.. If placed in the bottom (default) position the color can be chosen from the color picker popup. This is for firefox. And it is for GM and players. The behaviour still is a bug I think (maybe the position of the color picker popup ist "above" the avatar). But you can use the workaround, (1) placing tha avatar temporaly on bottom, (2) change the color and (3) place the avatar again on your preferred position.