I’m sure it depends on usage and the particular display design. I think the monochrome Palm I had was either a Palm V. It had a “backlight” that lights the part which is normally black. In complete darkness you get green letters on black background. In a well lit room, with the backlight off, you get black letters on light gray background. If the light level is somewhere in between, you get green letters on gray background - no contrast at all!
One of the 'net book sellers had a paperpack series in Ebook the same price as hardback. That’s silly. Sell 'em for .25 apiece and I’ll buy 100. $5.00 and I don’t buy any.