Not sure about the greyscale, but I agree with everything else he writes.
Consistency, words and pics, general to specific, all those are good practice IMO. I also ALWAYS use a specific, different color for HMI PBs that change screens so that there is never a question as to whether pressing *THAT* button will trigger something on the machine or in the cell vs. bringing up another screen.
Colors... well, I have to admit that I like to use green and red on my top-level screens. "Green Screen" I can see at a glance, from a distance, that nothing requires immediate attention. And, I often "grey out" equipment that is not available at that instant in the cell or on the machine. I have some fairly complicated process cells where I essentially replicated the PLC IKs on HMI pushbuttons. If the PB is aqua with black lettering, then pressing it will actuate the equipment or take some action. If grey with white lettering, no. This cell has enough options that *I* need that probably more than the OPRs since even though I programmed it, I don't work with it every day!
Anyway, it's an interesting article. I do tend to agree that a photo-realistic graphic of the system, or a straight copy of the ACAD P&ID, is not usually the best way to go. His grey-scale screen... is busy, but it's blocked out well. If it's showing the entire process or a significant unit, well OK. Like Jesper said, that's not something that you'd put on a 6-in screen. I hope! But from an engineering viewpoint, I like his layout of process info at the top, no pictograph. The picture or P&ID might be great on another screen to show you the location of the alarmed channel, or it's relationship to other stuff in the system, but to look at the P&ID with little numbers and such next to valves, flowmeters, pumps, tanks, etc yeah that can be slow to absorb.
I would still tend to use colors, but I agree that consistency and only a few are important. And, you need words to go with the colors given that some people are color blind and that standards can vary across industries.
The indicators making a pattern are good too... I don't have much process stuff (as that greyscale screen is showing) but it's a good idea. That is a lot better than just a numeric indicator on the screen, that maybe changes colors for H-L and HH-LL. If you had one or two process variables, not really a big deal either way, but if 10 or 20 or more, then I think his analog scales are a good idea. I don't mind the screen, it's full but if it gives me the top level view quickly and consistently, fine.