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!

Sad as it is, that is exactly what's happening..

SU Version: 15.3.331 64-bit on Win8.1
Printer: Epson TX510FN (inkjet printer)

I have a black outline (just lines, no fill) that I'm trying to print.
When printing in standard quality, the output is black and all is good.
But when trying to print in ultra/high-definition or HLR, the output is green instead of the expected black.

When trying to set the printer to print in grayscale nothing is being printed (blank/empty page).

I tried printing to PDF (CutePDF virtual printer) and then printing via Acrobate Reader, but the results are exactly the same as above (green lines or empty sheet in grayscale).

Any ideas on how to make SU print in black color as expected?