Guys, you were both spot on! Dave: Magenta is indeed low. TIG: Trying to print black lines via an image editor in grayscale mode (applied both to the image and printer settings) didn't print anything. After a bit of fiddling with the printer's settings I realized that, trying to make the lines print in a fine resolution, I selected the printer's "Text & Image" print quality. That was the problem. Changing the print quality to just "Text" made it possible to print the lines in grayscale mode with no color involved. Thanks a lot folks!