1. A computer-implemented method for generating a variable-density mono-color under-print layer corresponding to an image to be printed, the method comprising: receiving a printable image, the printable image comprising a plurality of first pixels; generating, by one or more processors, a mono-color under-print layer, the mono-color under-print layer comprising a plurality of second pixels, each second pixel corresponding to a first pixel in a same corresponding position in the printable image and each second pixel having an associated opacity whose value is based on at least one color characteristic of its corresponding first pixel, wherein the step of generating the mono-color under-print layer comprises determining a level of the at least one color characteristic of each first pixel and setting an opacity value of the corresponding second pixel to a value that is a function of the determined level when the determined level is within a pre-determined range, and setting the opacity value to a pre-determined constant opacity when the determined level is outside of the pre-determined range.