According to one example of the present invention, there is provided a method of generating data to control the ejection of ink drops from a plurality of arrays of printhead nozzles of a printing system. In the printing system one of the arrays of nozzles is configured to eject a first ink of a first color having a first colorant density. Another one of the arrays of nozzles is configured to eject a second ink of the first color having a second colorant density. Each array of nozzles is capable of making ink marks having a plurality of different ink mark volumes on a media. The method comprises obtaining image data for a single color channel and generating, from the image data, multi-level screen data describing, for each of the first and second inks, the ink mark volume to be made by the array of printhead nozzles at each screen location.