Introducing system color wrappers to benefit from the new system colors under iOS 12 and below.

UIColor+SystemColors.swift

//
//  UIColor+SystemColors.swift
//
//  https://gist.github.com/hoppsen/f3cbe2dd51ad40cfe80609b2e9bebada
//
//  More about Apple's System Colors can be found here:
//  https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/color/

import UIKit

// MARK: - Update existing colors to iOS 13 values

extension UIColor {
    static let systemGreenWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGreen
        } else {
            return UIColor(red: 52.0 / 255.0, green: 199.0 / 255.0, blue: 89.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemPurpleWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemPurple
        } else {
            return UIColor(red: 175.0 / 255.0, green: 82.0 / 255.0, blue: 222.0 / 255.0, alpha: 1.0)
        }
    }()
}

// MARK: - Make new colors available for iOS 12 and below

extension UIColor {
    static let systemIndigoWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemIndigo
        } else {
            return UIColor(red: 88.0 / 255.0, green: 86.0 / 255.0, blue: 214.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemGray2Wrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGray2
        } else {
            return UIColor(red: 174.0 / 255.0, green: 174.0 / 255.0, blue: 178.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemGray3Wrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGray3
        } else {
            return UIColor(red: 199.0 / 255.0, green: 199.0 / 255.0, blue: 204.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemGray4Wrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGray4
        } else {
            return UIColor(red: 209.0 / 255.0, green: 209.0 / 255.0, blue: 214.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemGray5Wrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGray5
        } else {
            return UIColor(red: 229.0 / 255.0, green: 229.0 / 255.0, blue: 234.0 / 255.0, alpha: 1.0)
        }
    }()

    static let systemGray6Wrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGray6
        } else {
            return UIColor(red: 242.0 / 255.0, green: 242.0 / 255.0, blue: 247.0 / 255.0, alpha: 1.0)
        }
    }()

    static let labelWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .label
        } else {
            return UIColor(white: 0.0, alpha: 1.0)
        }
    }()

    static let secondaryLabelWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .secondaryLabel
        } else {
            return UIColor(red: 0.235, green: 0.235, blue: 0.263, alpha: 0.6)
        }
    }()

    static let tertiaryLabelWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .tertiaryLabel
        } else {
            return UIColor(red: 0.235, green: 0.235, blue: 0.263, alpha: 0.3)
        }
    }()

    static let quaternaryLabelWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .quaternaryLabel
        } else {
            return UIColor(red: 0.235, green: 0.235, blue: 0.263, alpha: 0.18)
        }
    }()

    static let linkWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .link
        } else {
            return UIColor(red: 0.0, green: 0.478, blue: 1.0, alpha: 1.0)
        }
    }()

    static let placeholderTextWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .placeholderText
        } else {
            return UIColor(red: 0.235, green: 0.235, blue: 0.263, alpha: 0.3)
        }
    }()

    static let separatorWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .separator
        } else {
            return UIColor(red: 0.235, green: 0.235, blue: 0.263, alpha: 0.29)
        }
    }()

    static let opaqueSeparatorWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .opaqueSeparator
        } else {
            return UIColor(red: 0.776, green: 0.776, blue: 0.784, alpha: 1.0)
        }
    }()

    static let systemBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemBackground
        } else {
            return UIColor(white: 1.0, alpha: 1.0)
        }
    }()

    static let secondarySystemBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .secondarySystemBackground
        } else {
            return UIColor(red: 0.949, green: 0.949, blue: 0.969, alpha: 1.0)
        }
    }()

    static let tertiarySystemBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .tertiarySystemBackground
        } else {
            return UIColor(white: 1.0, alpha: 1.0)
        }
    }()

    static let systemGroupedBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemGroupedBackground
        } else {
            return UIColor(red: 0.949, green: 0.949, blue: 0.969, alpha: 1.0)
        }
    }()

    static let secondarySystemGroupedBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .secondarySystemGroupedBackground
        } else {
            return UIColor(white: 1.0, alpha: 1.0)
        }
    }()

    static let tertiarySystemGroupedBackgroundWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .tertiarySystemGroupedBackground
        } else {
            return UIColor(red: 0.949, green: 0.949, blue: 0.969, alpha: 1.0)
        }
    }()

    static let systemFillWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .systemFill
        } else {
            return UIColor(red: 0.471, green: 0.471, blue: 0.502, alpha: 0.2)
        }
    }()

    static let secondarySystemFillWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .secondarySystemFill
        } else {
            return UIColor(red: 0.471, green: 0.471, blue: 0.502, alpha: 0.16)
        }
    }()

    static let tertiarySystemFillWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .tertiarySystemFill
        } else {
            return UIColor(red: 0.463, green: 0.463, blue: 0.502, alpha: 0.12)
        }
    }()

    static let quaternarySystemFillWrapper: UIColor = {
        if #available(iOS 13.0, *) {
            return .quaternarySystemFill
        } else {
            return UIColor(red: 0.455, green: 0.455, blue: 0.502, alpha: 0.08)
        }
    }()
}