ContentView.swift

import ColorContrastRatio
import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            Text("Luminance for red color").padding()
            Rectangle()
                .frame(width: 50, height: 50)
                .fixedSize()
                .foregroundColor(.red)
            Text("\(UIColor.red.relativeLuminance)")
        }
    }
}