Most printing technologies still work with dots: dots of ink, dots of toner, dots of wax. You can get greyscale by varying the amount of stuff in a dot, which g

Most printing technologies still work with dots: dots of ink, dots of toner, dots of wax. You can get greyscale by varying the amount of stuff in a dot, which generally will make the dot bigger or smaller without changing its actual color. You can also get variations by using different colors of ink/toner/wax for different dots, or mixed in the same dot. For very-long-term archival, these variations are probably too subtle and too likely to be lost over time. A black and white pattern of dots will probably last a lot longer and be easier to interpret than slight variations in size and/or shade of the dots.