Color normalization

[melanieseltzer]

There's some lingering to-dos for semantic colors that we'll need to complete for the next release milestone.

From #377 we've identified:

• Look at the shades of blue and purple used, decide on primary and secondary colors.
• Go through the remaining RAW_COLORS and decide whether we want to add a new semantic color, make an exception, or better, change it to use an existing semantic color.
• Add a brief sentence or two in the Storybook explaining the purpose of semantic colors and when it's OK to deviate from them (only when an existing COLORS value absolutely won't work).

Anything else?