Im unable to create a set of colour variables and then store them in an array.

I had the same problem and solved it by moving all the initialization code into the init() function of the struct:

import Foundation
import UIKit

struct ColorWheel {

    let colors: [UIColor]

    init() {
        let teal     = UIColor(red:  90/255.0, green: 187/255.0, blue: 181/255.0, alpha: 1.0)
        let yellow   = UIColor(red: 222/255.0, green: 171/255.0, blue:  66/255.0, alpha: 1.0)
        let red      = UIColor(red: 223/255.0, green:  86/255.0, blue:  94/255.0, alpha: 1.0)
        let orange   = UIColor(red: 239/255.0, green: 130/255.0, blue: 100/255.0, alpha: 1.0)
        let darkGray = UIColor(red:  77/255.0, green:  75/255.0, blue:  82/255.0, alpha: 1.0)
        let purple   = UIColor(red: 105/255.0, green:  94/255.0, blue: 133/255.0, alpha: 1.0)
        let green    = UIColor(red:  85/255.0, green: 176/255.0, blue: 112/255.0, alpha: 1.0)

        colors = [teal, yellow, red, orange, darkGray, purple, green]
    }

}

These individual color variables are not visible outside the init() function and the ColorWheel struct has only one member: colors. When you do not put the color constants inside the init() function, they are not local variables but members of the struct (e.g. you can access ColorWheel.red from outside the struct). When I understand the error message correctly, you cannot set values of member variables by using other member variables during initialization, but I have to read up the details in the The Swift Programming Language iBook (it provides a lot of details about structs).

My motivation to rewrite the code was to get rid of the comments and use variable names instead. As I am using let there should not be any performance implications, but I'd like to get feedback about that.