The code transforms color codes which are 3 digits like "#000", 8 digits (with opacity) or named colors to a valid 6 digit hex code, using System.Drawing

ColorCodeTransform

public static string ToHexadecimalColorCode(string color)
{
    Color baseColor = ColorTranslator.FromHtml(color);
    
    // if its a named color we can use the argb value to transform it to a hex
    var argb = baseColor.ToArgb();
    return ColorTranslator.ToHtml(Color.FromArgb(argb));
}