using System;

using System.Drawing;

using System.Linq;

​

var colors = Enum.GetValues<KnownColor>()

    .Select(Color.FromKnownColor)

    .Where(x => x.IsNamedColor && !x.IsSystemColor && x.IsKnownColor)

    .Select(x => x.Name)

    .Distinct()

    .ToList();

​

var randomColors = colors.OrderBy(x => Guid.NewGuid()).Take(3);

​

foreach (var color in randomColors)

{

    Console.WriteLine(color);

}

​