After Clicked Button its Colour Need to Change

Question

Hi Friends.....,

Hi i Had Created One Page in That i Assigned Four Panels (Using UpdatePanel)

Panel A , Panel B , Panel C , Panel D 

For Each and Every Panel There is Button For eg: If Button1 Clicks It Will Show Panel A Same as Button2 Clicks It Will Show Panel B

In Css I Had Created Hover When Placing My Mouse Pointer Its Changing Color Fine ,But Now i Need to Change Color After Button Clicked or Selected

How it Needs to Work is IF " i Click on Button1 Panel A Will be in Open at the Same time, Button1 Need to Be in Another Color "

Is their Any Code or Link For That Previously I Had Used 

Button1.Attributes["onclick"] = "this.style.backgroundColor='red';";
Button1.BackColor = System.Drawing.Color.Red;

on Onclick of Button1_Click in .cs FIle

Its Not Working For Me

With Regards....,
R.V.Rajagopalan