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You should barely be able to see a difference between the inner and outer color. If you're on a laptop also make sure to adjust the viewing angle of your screen. Don't worry if you can't see all the colors, the game has built-in tolerance and will give your a perfect score if the colors are very similar.
We implemented a solution for players with color perception deficiencies: each primary color represents a shape, and the intermediate steps are represented by a morph between each shape. The shape is meant as a general guide to pick the right hue, you will still need to adjust the color to get a good score. Let us know how it works out for you at email@example.com.