Q:

PDF Watermark for printing only, programmatically

I can watermark any PDF already, and the images inside, everything ok, but now I need the watermark only showing up when the PDF is printed... Is this possible?  How?  
I need to do this programmatically of course. 

A:

For future readers, this is possible to do by wrapping the watermark in a PDF layer (Optional Content Group), then configuring the Usage attribute of this layer as Print-Only.  See the PDF Reference Document, Chapter 4-Graphics, part 4.10-Optional Content for more details. 

A:

You should probably make use of the fact that the screen uses RGB and the printer CMYK.  You should be able to create two colors in CMYK that map to the same RGB value.  This is of course not enough against a determined specialist.