private void button_Click(object sender, EventArgs e)
{
// Mouse up first
button_MouseUp(sender, new MouseEventArgs(MouseButtons.Left, 1, 0, 0, 0));
// Show color dialog
dialog.Color = panel.BackColor;
if(dialog.ShowDialog(this.ParentForm) == DialogResult.OK)
{
// Apply new color
panel.BackColor = dialog.Color;
}
}