使用System.Drawing.Color类型的可选参数

public class MyObject 
{
    public Color Rgb { get; private set; }

    public MyObject(int foo, string bar, Color? rgb = null) 
    { 
        this.Rgb = rgb ?? Color.Transparent;
        // .... 
    } 
}

static readonly 直到运行时才设置值(由类型初始值设定项)

不要创建可选参数,而是像这样重载函数:

public myObject(int foo, string bar) : this (foo, bar, Color.Transparent) {};

public myObject(int foo, string bar, Color RGB) {
...
}

public myObject(int foo, string bar, Color rgb = default) {
    // ....
}

试试这个:

public myObject(int foo, string bar, string colorName = "Transparent")
{
    using (Pen pen = new Pen(Color.FromName(colorName))) //right here you need your color
    {
       ///enter code here
    }

}