How is this Widget changing the colour of an image?

Hey guys,

My client wants me to create something similar to the ‘colored widget’ shown here:

Initially I thought this seems like a fairly straight forward project, but then I noticed the button is not a button at all, but an image!

So my question is, how exactly is this possible? I notice the button works in IE7 so it cant be any HTML5 trickery?

How would I go about replicating this with JavaScript ? P.S I’m experienced with JS so get as technical as you like :slight_smile:
thanks in advance!! :slight_smile: