Changing the color of a drawn rectangle

Hi Experts,

   I have a user control (C# 2005 w/ .NET FW 2.0), which has a picture box. When the user control gets created for the first time, I draw a rectangle on top of it using the picture box' Graphics object and the FillRectangle (Brush, RectangleF) function (using a SolidBrush).  I have a public method, that's supposed to allow the user to change the rectangle's color.  My question is: How do I change the color of that existing rectangle? I tried saving the RectangleF object and just calling the FillRectangle function on it a second time using a different Brush color, but that returns a "Parameter is not valid" error.

Any help would be greatly appreciated.

Thanks,
axnst2