Apparently the LCD in the 540 is a true 6-bit panel.

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That doesn't actually surprise me. 6 bit grey resolution was what Mono VGA was capable of, it would make sense they'd manufacture panels that support it.

I'm sure with the color display the control panel says "256 greys". I would just wager that in practice they're dithered, not true.

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What's interesting about this is that it means that the CLUT register is capable of either 256 colors (3.3.2.0.0), or, at a minimum, a 6-bit linear grayscale value (6.0.0). Considering grayscale values larger than a single color channel are possible in the register, technically, it should be possible for it to output true 256 grayscale values, with a true 8-bit grayscale panel attached to it. Of course, at that time, the most one ever saw was 6-bit panels. However, it would be fun to find a true 8-bit grayscale panel and see if the CLUT could drive it at the full 8-bit range. The OS is capable of handling 256 grays, so I don't see why not. A fun little hack to try. Unfortunately, far beyond my capabilities.