spectre.console/examples/Shared/Extensions/ColorExtensions.cs

namespace Spectre.Console.Examples
{
    public static class ColorExtensions
    {
        public static Color GetInvertedColor(this Color color)
        {
            return GetLuminance(color) < 140 ? Color.White : Color.Black;
        }

        private static float GetLuminance(this Color color)
        {
            return (float)((0.2126 * color.R) + (0.7152 * color.G) + (0.0722 * color.B));
        }
    }
}