tailwindcss/src/plugins/fill.js

import _ from 'lodash'
import flattenColorPalette from '../util/flattenColorPalette'

export default function() {
  return function({ addUtilities, e, theme, variants }) {
    const colors = flattenColorPalette(theme('fill'))

    const utilities = _.fromPairs(
      _.map(colors, (value, modifier) => {
        return [`.${e(`fill-${modifier}`)}`, { fill: value }]
      })
    )

    addUtilities(utilities, variants('fill'))
  }
}