UIColor extension to be used with the generated file representing the asset catalog parsed colors.

UIColor+DefaultColors.swift

import UIKit

extension UIColor {
    static func defaultColor(named name: String, default defaultColor: UIColor = .black) -> UIColor {
        if #available(iOS 11, *) {
            return UIColor(named: name) ?? defaultColor
        }
        else {
            return _defaultColors[name] ?? defaultColor
        }
    }
}