engineSwitcher.EngineFactories
    .AddYantra(new YantraSettings
    {
        ConsoleCallback = (string type, object[] args) =>
        {
            TextWriter writer = type == "error" ? Console.Error : Console.Out;
            if (type != "log")
            {
                ConsoleColor typeColor;
                switch (type)
                {
                    case "warn":
                        typeColor = ConsoleColor.Yellow;
                        break;
                    case "error":
                        typeColor = ConsoleColor.Red;
                        break;
                    default:
                        typeColor = ConsoleColor.White;
                        break;
                }
                Console.ForegroundColor = typeColor;
                writer.Write("{0}: ", type);
            }
            Console.ForegroundColor = ConsoleColor.White;
            for (int argIndex = 0; argIndex < args.Length; argIndex++)
            {
                if (argIndex > 0)
                {
                    writer.Write(" ");
                }
                writer.Write(args[argIndex] ?? "null");
            }
            writer.WriteLine();
        }
    })
    ;

IJsEngine engine = new YantraJsEngine(
    new YantraSettings
    {
        ConsoleCallback = (string type, object[] args) =>
        {
            …
        }
    }
);