For Window Form application :
private void button3_Click(object sender, EventArgs e)
{
button3.BackColor = System.Drawing.Color.Black;
}
For Web Page :
On Page level declare static variable:
static bool Isdefault= true;
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Isdefault = false;
}
}
and in button click event write :
protected void Button1_Click(object sender, EventArgs e)
{
if (!Isdefault)
{
Button1.BackColor = System.Drawing.Color.GreenYellow;
}
}