Problem using GUI's drawRect during RenderGameOverlayEvent

I'm trying to render some information on a bar at the top of the screen.

At the moment I'm just trying to get a single color rectangle to be rendered and make it scale nicely into the position I want it before I start adding text and icons to it.

So during a RenderGameOverlayEvent I do the following:

if (event.isCancelable() || event.type != ElementType.EXPERIENCE) {
            return;
drawRect(10, 10, 100, 100, 0x383838);

I have checked that the drawRect line is being run, but nothing happens on the screen. Which means no effect what so ever.

What reason could this be? I did use drawTexturedModalRect() method in my other mod and it worked perfectly while being called from the same kind of position.