Error: Cannot assign to 'backgroundColor' in 'self'

struct Playlist {
    let title: String?
    let description: String?
    let icon: UIImage?
    let largeIcon: UIImage?
    let artists: [String] = []
    let backgroundColor = UIColor.clearColor()

    init(index: Int){
        let musicLibrary = MusicLibrary().library
        let playlistDictionary = musicLibrary[index]

        title = playlistDictionary["title"] as! String!
        description = playlistDictionary["dictionary"] as! String!
        let iconName = playlistDictionary["icon"] as! String!
        icon = UIImage(named: iconName)
        let largeIconName = playlistDictionary["largeIcon"] as! String!
        largeIcon = UIImage(named: largeIconName)

        artists += playlistDictionary["artists"] as! [String]

        let colorsDictionary = playlistDictionary["backgroundColor"] as! [String : CGFloat]

        backgroundColor = rgbColorFromDictionary(colorsDictionary)

        }

    func rgbColorFromDictionary (colorDictionary: [String:  CGFloat]) -> UIColor {
        let red = colorDictionary["red"]!
        let blue = colorDictionary["blue"]!
        let green = colorDictionary["green"]!
        let alpha = colorDictionary["alph"]!

        return UIColor(red: red/255.0, green: green/255.0, blue:  blue/255.0, alpha: alpha)

    }

}