You just need to flip the color based on its current value. That can be done by doing:
label1.BackColor = label1.BackColor == Color.Red ? Color.Transparent : Color.Red;
The above is a conditional operator
and is basically just shorthand for an if/else statement,
if (label1.BackColor == Color.Red)
label1.BackColor = Color.Transparent
else
label1.BackColor = Color.Red;