C#

Color original = Color.Yellow;
Color opposite = Color.FromArgb(255 - original.R, 255 - original.G, 255 - original.B);<pre>

bool ToChange = true;
string OldText = "";

private void TextBox_KeyDownHandler(object sender, KeyDownArgs e)
{
    If(e.KeyChar == "\b")
    {
        OldText = TextBox.Text;
        ToChange = false;
    }
    else
        ToChange = true;
}

private void TextBox_TextChanged((object sender, KeyDownArgs e)
{
    If(!ToChange)
    {
        TextBox.Text = OldText;
        TextBox.SelectionStart = TextBox.TextLength;
    }
}

private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
	if(e.KeyChar == (char)Keys.Back)
		e.Handled = true;
}