1. A processor-implemented method for processing image data using an image processing apparatus, the image processing apparatus comprising at least one processor for processing documents containing image data comprising a plurality of pixels, the method comprising the following acts implemented by the at least one processor: receiving image data of a document comprising a plurality of pixels; determining a billable count of color pixels in the received image data by: determining a first count of color pixels in the image data in device independent space; converting the plurality of pixels of the image data into device dependent space; determining a second count of color pixels for marking in the device dependent image data; transforming the determined first and second counts of color pixels, and determining a minimum of the two transformed pixel counts.