Propagation of map styles from colors

[mariogiampieri]

Currently, colors.ts has an array of hex codes that are used to generate the map ui color picker. There is a separate and verbose ExpressionSpecification in layers.ts which uses a hard-coded copy of that array. I would like to have that ExpressionSpecification be generated dynamically so we don't have to have two copies of those colors if/when they change over time.