Photoshop :: Difficulty With Brush Tool / Restore Orange Color

having some difficulty with the brush tool. I am currently taking web design with the centre for distance education and am trying to do a fairly basic assignment with photoshop. So, I am being asked to first create a layer with an orange fill to it. Then I am asked to create a layer mask, I am then asked to put an image behind the orange which I am then asked to use the brush tool with black to see through the orange colour and through to the image. I am having no problem with this until I am asked to change the brush colour to white and go over the previously bushed areas and restore the orange colour. When I do this nothing happens.