Yesterday the customer brought a sheet of paper with some colour on it, and said that he wants the cover photo for his booklet to be exact the same. The quickest way to do that turned to be to convert the photo (it was in CMYK already) to bw, invert it, make a regular (not adjustment) layer, fill it with the colour and set the blending mode to exclusion. Then to add curves layer to get the proper contrast. Other methods were not as clean and fast. Any other methods you use?