In this scenario, I need to read the color of pixel x,y on my 800x600x32 .png background image
If this was because they clicked rectangle which is a certain colour, your Rectangle type could contain a colour field. then when they click the mouse, you work out which rectangle is clicked via the coords (and see which is uppermost if they overlap) and then when you know which one, simply read the colour field. No Pixmaps involved :-)