UIColor extension to generate a random pastel

RandomPastelUIColor.swift

import UIKit

public extension UIColor {

    private static var randomLightness: CGFloat {
        Double.random(in: 0...1)
    }

    private static var randomRGB: UIColor {
        UIColor(
            red: randomLightness,
            green: randomLightness,
            blue: randomLightness,
            alpha: 1
        )

    }

    /// Generate a random Pastel color.
    ///
    /// Wikipedia:
    ///
    /// *'Pastels belong to a pale family of colors, which, when described in the HSV color space, have high value and low saturation.'*
    static var randomPastel: UIColor {

        // 'HSB' and 'HSV' are different names for the same color spaces defined by:
        // * Hue: the tint of the color.
        // * Saturation: the distance of the color from a pure grey of the same Brightness.
        // * Brightness/Value: How 'lit up' the color is. 0 = Black. 1 = 'Fully visible'.

        // We define 'Pastel' as:
        // * Any hue...
        // * ...of low saturation [0.05, 0.4]...
        // * ...and moderate to high brightness [0.6, 1.0].

        // Generating a random number from zero to one in the HSB color space would not
        // appear 'random' to humans. It would appear to show more of certain colors as
        // they are 'stretched' in the space.
        // See: https://en.wikipedia.org/wiki/HSL_and_HSV

        // The RGB color space is also not uniform across human perception. We would
        // prefer a random base color in the CGColorSpace.genericLab color space which
        // is modelled to be uniform across human perception.
        // However the L*a*b* color space has invalid regions when sampled naively — so
        // randomization is harder.
        // We stick to RGB.

        let rgb = randomRGB

        // Extract the sample's HSB/HSV values.
        var hue: CGFloat = 0
        var saturation: CGFloat = 0
        var brightness: CGFloat = 0
        var alpha: CGFloat = 0
        rgb.getHue(&hue, saturation: &saturation, brightness: &brightness, alpha: &alpha)

        let pastelHue = hue
        let pastelSaturation = 0.05 + (saturation * 0.35)
        let pastelBrightness = 0.6 + (brightness * 0.4)

        return UIColor(
            hue: pastelHue,
            saturation: pastelSaturation,
            brightness: pastelBrightness,
            alpha: 1
        )
    }


}