Return a color based on percentage. 0% is Red, and 100% is Green

DynamicColors.swift

  extension UIColor {
    static func colorForPercent(_ percentage: Double) -> UIColor {
      switch percentage {
      case 0...0.5:
        return firstHalf(percentage: percentage * 2)
      case 0.5...1:
        return secondHalf(percentage: (percentage - 0.5) * 2)
      default:
        fatalError("Bad Input: percentage must satisfy 0 <= percentage <= 1")
      }
    }

    static fileprivate func firstHalf(percentage: Double) -> UIColor {
      let begin = UIColor.systemGreen.cgColor.components!
      let end = UIColor.systemYellow.cgColor.components!
      return createColor(begin: begin, end: end, percentage: percentage)
    }

    static fileprivate func secondHalf(percentage: Double) -> UIColor {
      let begin = UIColor.systemYellow.cgColor.components!
      let end = UIColor.systemRed.cgColor.components!
      return createColor(begin: begin, end: end, percentage: percentage)
    }

    static fileprivate func createColor(begin: [CGFloat], end: [CGFloat], percentage: Double) -> UIColor {
      var colorDiff = begin
      for i in 0 ..< 3 {
        colorDiff[i] = begin[i] + (end[i] - begin[i]) * CGFloat(percentage)
      }
      return UIColor(red: colorDiff[0], green: colorDiff[1], blue: colorDiff[2], alpha: 1)
    }
}