Using UIColors from iOS13 with backward compatibility to previous iOS versions.

ColorSupport.swift

import UIKit

public enum ColorSupport {
    public static var label: UIColor {
        if #available(iOS 13, *) {
            return .label
        }
        return UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 1.0)
    }

    public static var secondaryLabel: UIColor {
        if #available(iOS 13, *) {
            return .secondaryLabel
        }
        return UIColor(red: 60.0 / 255.0, green: 60.0 / 255.0, blue: 67.0 / 255.0, alpha: 0.6)
    }

    public static var tertiaryLabel: UIColor {
        if #available(iOS 13, *) {
            return .tertiaryLabel
        }
        return UIColor(red: 60.0 / 255.0, green: 60.0 / 255.0, blue: 67.0 / 255.0, alpha: 0.3)
    }

    public static var quaternaryLabel: UIColor {
        if #available(iOS 13, *) {
            return .quaternaryLabel
        }
        return UIColor(red: 60.0 / 255.0, green: 60.0 / 255.0, blue: 67.0 / 255.0, alpha: 0.18)
    }

    public static var systemFill: UIColor {
        if #available(iOS 13, *) {
            return .systemFill
        }
        return UIColor(red: 120.0 / 255.0, green: 120.0 / 255.0, blue: 128.0 / 255.0, alpha: 0.2)
    }

    public static var secondarySystemFill: UIColor {
        if #available(iOS 13, *) {
            return .secondarySystemFill
        }
        return UIColor(red: 120.0 / 255.0, green: 120.0 / 255.0, blue: 128.0 / 255.0, alpha: 0.16)
    }

    public static var tertiarySystemFill: UIColor {
        if #available(iOS 13, *) {
            return .tertiarySystemFill
        }
        return UIColor(red: 118.0 / 255.0, green: 118.0 / 255.0, blue: 128.0 / 255.0, alpha: 0.12)
    }

    public static var quaternarySystemFill: UIColor {
        if #available(iOS 13, *) {
            return .quaternarySystemFill
        }
        return UIColor(red: 116.0 / 255.0, green: 116.0 / 255.0, blue: 128.0 / 255.0, alpha: 0.08)
    }

    public static var placeholderText: UIColor {
        if #available(iOS 13, *) {
            return .placeholderText
        }
        return UIColor(red: 60.0 / 255.0, green: 60.0 / 255.0, blue: 67.0 / 255.0, alpha: 0.3)
    }

    public static var systemBackground: UIColor {
        if #available(iOS 13, *) {
            return .systemBackground
        }
        return UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
    }

    public static var secondarySystemBackground: UIColor {
        if #available(iOS 13, *) {
            return .secondarySystemBackground
        }
        return UIColor(red: 242.0 / 255.0, green: 242.0 / 255.0, blue: 247.0 / 255.0, alpha: 1.0)
    }

    public static var tertiarySystemBackground: UIColor {
        if #available(iOS 13, *) {
            return .tertiarySystemBackground
        }
        return UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
    }

    public static var systemGroupedBackground: UIColor {
        if #available(iOS 13, *) {
            return .systemGroupedBackground
        }
        return UIColor(red: 242.0 / 255.0, green: 242.0 / 255, blue: 247.0 / 255.0, alpha: 1.0)
    }

    public static var secondarySystemGroupedBackground: UIColor {
        if #available(iOS 13, *) {
            return .secondarySystemGroupedBackground
        }
        return UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
    }

    public static var tertiarySystemGroupedBackground: UIColor {
        if #available(iOS 13, *) {
            return .tertiarySystemGroupedBackground
        }
        return UIColor(red: 242.0 / 255.0, green: 242.0 / 255, blue: 247.0 / 255.0, alpha: 1.0)
    }

    public static var separator: UIColor {
        if #available(iOS 13, *) {
            return .separator
        }
        return UIColor(red: 60.0 / 255.0, green: 60.0 / 255.0, blue: 67.0 / 255.0, alpha: 0.29)
    }

    public static var opaqueSeparator: UIColor {
        if #available(iOS 13, *) {
            return .opaqueSeparator
        }
        return UIColor(red: 198.0 / 255.0, green: 198.0 / 255.0, blue: 200.0 / 255.0, alpha: 1.0)
    }

    public static var link: UIColor {
        if #available(iOS 13, *) {
            return .link
        }
        return UIColor(red: 0.0, green: 122.0 / 255.0, blue: 1.0, alpha: 1.0)
    }

    public static var darkText: UIColor {
        if #available(iOS 13, *) {
            return .link
        }
        return UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 1.0)
    }

    public static var lightText: UIColor {
        if #available(iOS 13, *) {
            return .link
        }
        return UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 0.6)
    }

    public static var systemBlue: UIColor {
        if #available(iOS 13, *) {
            return .systemBlue
        }
        return UIColor(red: 0.0, green: 122.0 / 255.0, blue: 1.0, alpha: 1.0)
    }

    public static var systemGreen: UIColor {
        if #available(iOS 13, *) {
            return .systemGreen
        }
        return UIColor(red: 52.0 / 255.0, green: 199.0 / 255.0, blue: 89.0 / 255.0, alpha: 1.0)
    }

    public static var systemIndigo: UIColor {
        if #available(iOS 13, *) {
            return .systemIndigo
        }
        return UIColor(red: 88.0 / 255.0, green: 86.0 / 255.0, blue: 214.0 / 255.0, alpha: 1.0)
    }

    public static var systemOrange: UIColor {
        if #available(iOS 13, *) {
            return .systemOrange
        }
        return UIColor(red: 1.0, green: 149.0 / 255.0, blue: 0.0, alpha: 1.0)
    }

    public static var systemPink: UIColor {
        if #available(iOS 13, *) {
            return .systemPink
        }
        return UIColor(red: 1.0, green: 45.0 / 255.0, blue: 85.0 / 255.0, alpha: 1.0)
    }

    public static var systemPurple: UIColor {
        if #available(iOS 13, *) {
            return .systemPurple
        }
        return UIColor(red: 175.0 / 255.0, green: 82.0 / 255.0, blue: 222.0 / 255.0, alpha: 1.0)
    }

    public static var systemRed: UIColor {
        if #available(iOS 13, *) {
            return .systemRed
        }
        return UIColor(red: 1.0, green: 59.0 / 255.0, blue: 48.0 / 255.0, alpha: 1.0)
    }

    public static var systemTeal: UIColor {
        if #available(iOS 13, *) {
            return .systemTeal
        }
        return UIColor(red: 90.0 / 255.0, green: 200.0 / 255.0, blue: 250.0 / 255.0, alpha: 1.0)
    }

    public static var systemYellow: UIColor {
        if #available(iOS 13, *) {
            return .systemYellow
        }
        return UIColor(red: 1.0, green: 204.0 / 255.0, blue: 0.0, alpha: 1.0)
    }

    public static var systemGray: UIColor {
        if #available(iOS 13, *) {
            return .systemGray
        }
        return UIColor(red: 142.0 / 255.0, green: 142.0 / 255.0, blue: 147.0 / 255.0, alpha: 1.0)
    }

    public static var systemGray2: UIColor {
        if #available(iOS 13, *) {
            return .systemGray2
        }
        return UIColor(red: 174.0 / 255.0, green: 174.0 / 255.0, blue: 178.0 / 255.0, alpha: 1.0)
    }

    public static var systemGray3: UIColor {
        if #available(iOS 13, *) {
            return .systemGray3
        }
        return UIColor(red: 199.0 / 255.0, green: 199.0 / 255.0, blue: 204.0 / 255.0, alpha: 1.0)
    }

    public static var systemGray4: UIColor {
        if #available(iOS 13, *) {
            return .systemGray4
        }
        return UIColor(red: 209.0 / 255.0, green: 209.0 / 255.0, blue: 214.0 / 255.0, alpha: 1.0)
    }

    public static var systemGray5: UIColor {
        if #available(iOS 13, *) {
            return .systemGray5
        }
        return UIColor(red: 229.0 / 255.0, green: 229.0 / 255.0, blue: 234.0 / 255.0, alpha: 1.0)
    }

    public static var systemGray6: UIColor {
        if #available(iOS 13, *) {
            return .systemGray6
        }
        return UIColor(red: 242.0 / 255.0, green: 242.0 / 255.0, blue: 247.0 / 255.0, alpha: 1.0)
    }
}

To use simply add this to your project and use instead of UIColor
For example:
replace UIColor.systemRed
with ColorSupport.systemRed