Provide color suggestions

[toolness]

It'd be nice if we could show color suggestions, to allow the palette to be "tweaked" to be more accessible.

This is pretty open-ended. Ideas include:

• Being able to "lock" certain colors from being auto-tweaked.
• Showing designers a list of more accessible choices, so that they can pick one that looks the most aesthetically pleasing.

We could potentially look at a similar feature in the Chrome Accessibility Developer Tools (and even borrow it, since it seems to be open-source).