There's a color, gray95, that is the recommended black and white color for watermarks in a few books I've read on the topic. Honestly the best way to do it right is to ask the court (politely) what their stated requirements for watermarking are and if you get shrugs, you go for the bare minimum.
there is no court where I have ever practiced that would accept a watermark of any kind at whatever percentage or color. And he didn't need to ask, it's right there in the rules (state and local). Every court has extremely detailed requirements for font, size, line spacing, line numbering, color of cover for printed "chambers copies," size of margins, how the name of the court should be set out and where, and so on. Literally no excuse for this, he's lucky he didn't get sanctioned
What a fascinating cultural difference. In my corner of the world there's a Facebook page, the name of which translates to "Half-assed court documents" and it showcases badly made documents issued by courts.