That’S the problem, you can just change the colour in a very limited way -> e.g when you put the little circle to a value between blue and pink then the inner radius will be blue and the outer radius a mixture between blue + pink.
But I would recommend you to try out what suggested. ^^ But my way also works, but of course not so well 
e.g basically at the picture above you see a bright yellow/orange in the inner part and a darker orange in the outer -> when you do the same with yellow + green you get a better result (yellow in the middle - green in the outer radius)