In Day 2 optional reading related to power of String interpolation. I had issues running the last code related to ColoredString because UIColor and UIFont are resolved in playground. How can I resolved that?

struct ColoredString: ExpressibleByStringInterpolation {
    // this nested struct is our scratch pad that assembles an attributed string from various interpolations
    struct StringInterpolation: StringInterpolationProtocol {
        // this is where we store the attributed string as we're building it
        var output = NSMutableAttributedString()

        // some default attribute to use for text
        var baseAttributes: [NSAttributedString.Key: Any] = [.font: UIFont(name: "Georgia-Italic", size: 64) ?? .systemFont(ofSize: 64), .foregroundColor: UIColor.black]

        // this initializer is required, and can be used as a performance optimization
        init(literalCapacity: Int, interpolationCount: Int) { }

        // called when we need to append some raw text
        mutating func appendLiteral(_ literal: String) {
            // print it out so you can see how it's called at runtime
            print("Appending \(literal)")

            // give it our base styling
            let attributedString = NSAttributedString(string: literal, attributes: baseAttributes)

            // add it to our scratchpad string
            output.append(attributedString)
        }

        // called when we need to append a colored message to our string
        mutating func appendInterpolation(message: String, color: UIColor) {
            // print it out again
            print("Appending \(message)")

            // take a copy of our base attributes and apply the color
            var coloredAttributes = baseAttributes
            coloredAttributes[.foregroundColor] = color

            // wrap it in a new attributed string and add it to our scratchpad
            let attributedString = NSAttributedString(string: message, attributes: coloredAttributes)
            output.append(attributedString)
        }
    }

    // the final attributed string, once all interpolations have finished
    let value: NSAttributedString

    // create an instance from a literal string
    init(stringLiteral value: String) {
        self.value = NSAttributedString(string: value)
    }

    // create an instance from an interpolated string
    init(stringInterpolation: StringInterpolation) {
        self.value = stringInterpolation.output
    }
}

// now try it out!
let str: ColoredString = "\(message: "Red", color: .red), \(message: "White", color: .white), \(message: "Blue", color: .blue)"

Did you import UIKit?

@rooterboy Yes, I did.

If you look at the section on the right side of the playground, and hover your mouse over the line where it is showing your strings, a little eye icon should appear next to it. If you tap on the eye, it should show you what it will actually look like with the color.