There may be fancier ways to do this, but you read the file in as an image and then use "unflatten pixmap.vi" to get a 2D array of all the image pixels. For example, indexing an element out of this array and feeding it into a color box indicate will show you the colour of the pixel.
You can then sort through this array counting all elements of a particular colour or within a particular range of colours, giving you all the info you need for your histogram. It will also allow you to compare different areas of the image by comparing different 'areas' of the 2D array.
Ian