Next, introduce a Levels-channel to the background by clicking the channels-button and scrolling to levels. With the options that pops up you can alter the levels of the underlying pixels without making definitive changes.
Drag the black-value slider to the right until it is at level 42 or 43. When you slide, you will see the "wall" around the window update to a darker grey, while the windows stays pure white. Click OK and save this new document as 5.psd :
Next, drag the slider to a position of 84 to 85 and save as 6.psd