I found colour matching for the radial gradient to be the most challenging. I ended up mostly eyeballing the difference between the general background and the background of the feedback area. I guess having a calibrated monitor would help, but apart from that, I would appreciate any tips dealing with colours. Eg. how much can I deviate from the guidelines?
Also I ended up making the submission button active only after the user has selected a rating. It was a deliberate solution and I would appreciate any feedback regarding it.