An extension of the iOS class UIColor to provide conversion to and from CIE xyY colors.

UIColor+CIExyY.swift

/// An extension to provide conversion to and from CIE xyY colors.
extension UIColor {

    /// The CIE xyY components of a color - luminance (Y) and chromaticity (x,y).
    struct CIExyY: Hashable {

        /// The x-axis chromaticity coordinate of the color, in the range [0, 1].
        var x: CGFloat
        /// The y-axis chromaticity coordinate of the color, in the range [0, 1].
        var y: CGFloat
        /// The luminance component of the color, in the range [0, 100].
        var Y: CGFloat

    }

    /// The CIE xyY components of the color.
    var xyY: CIExyY {
        let XYZ = self.XYZ
        let sum = XYZ.X + XYZ.Y + XYZ.Z

        guard sum != 0.0 else {
            return CIExyY(x: 0.0, y: 0.0, Y: XYZ.Y)
        }

        let x = XYZ.X / sum
        let y = XYZ.Y / sum

        return CIExyY(x: x, y: y, Y: XYZ.Y)
    }

    /// Initializes a color from CIE xyY components.
    /// - parameter xyY: The components used to initialize the color.
    /// - parameter alpha: The alpha value of the color.
    convenience init(_ xyY: CIExyY, alpha: CGFloat = 1.0) {
        guard xyY.y != 0.0 else {
            self.init(CIEXYZ(X: 0.0, Y: xyY.Y, Z: 0.0), alpha: alpha)
            return
        }

        let X = (xyY.x * xyY.Y) / xyY.y
        let Y = xyY.Y
        let Z = ((1.0 - xyY.x - xyY.y) * xyY.Y) / xyY.y

        self.init(CIEXYZ(X: X, Y: Y, Z: Z), alpha: alpha)
    }

}

This requires the CIEXYZ extension declared at https://gist.github.com/adamgraham/677c0c41901f3eafb441951de9bc914c