How to get the RGB values of each pixel in the windows form with GDI+ ?

Question

Hi all, I'm new to GDI+ and C#, Recently, I try to implement a painer, I want to get each pixel's RGB values in the form when I click the mouse. In the MouseDown function, I have tried "this.GetChildAtPoint(pt).ForeColor.ToArgb();", but "Exception Error!!" comes up. How should I solve this problem? Thanks... :)