Toggle colour change on click

[KeredDrahcir]

I have a division and I want to click on it to change the colour to red and click on it again to change the colour back to black.
The code I'm using doesn't seem to work and I can't work out what I'm doing wrong. Can anyone help me?

HTML

<div style="width: 30px; height: 30px; border-style: solid; border-color: #ffffff; border-width: 2px 0 0 2px; float: left; background-color: #000000;" id="a1" onclick="plan('a1');">&nbsp;</div>

JavaScript

function plan(id){
    var obj = document.getElementById(id);
    (obj.style.backgroundColor == "#000000") ? obj.style.backgroundColor = "#ff0000" : obj.style.backgroundColor = "#000000";
}

Many thanks.

[yolshevsky]

This works.

obj.style.backgroundColor = (obj.style.backgroundColor == "rgb(0, 0, 0)") ? "#ff0000" : "#000000";
 

apparently the background property is not a string.

[KeredDrahcir]

Fantastic. Works perfectly. Many thanks.