I'm 100% certain that the Samsung 225BW (and up) is 8 bit per color though ...
I'm not familiar with current LCDs but BeHardware (aka Hardware.fr, which offers a lot of in-depth reviews) seems to think it's 6-bit.
I haven't read it, but they also have an article on the 226BW (and its two variants), which might contain further info.
I was under the impression that TN panels were inherently 6-bit and poor at off-angle viewing, but this Acer is listed as 16.7M colors. It's possible color range and viewing angle are separate issues WRT panel type. OTOH, it's also possible manufacturers are rendering the distinction b/w 6- and 8-bit panels moot with creative labelling thanks to improved dithering (they called the old TN panels 16.2M, so if they've since reduced perceptible dithering they may consider it acceptable to boost the colors figure).
Wikipedia says "most [TN displays] are unable to display the full 16.7 million colors (24-bit truecolor)," so it's possible some of these newer TN panels are breaking from their value roots and offering higher color depth.
An article at TFT Central (referenced by the Wikipedia entry) says that "TN Film screens are traditionally more economical. In fact, they only display 64 red, 64 blue and 64 green shades," BUT a link in that very section points to another (presumably newer) article on their site, 6-Bit vs. 8-Bit, that says:
The reason that TN Film panels have traditionally been 6 bit panels is that the manufacturers used low performance A/D Convertors. This was a means to keep costs of these panels down, and also to help improve responsiveness. That's why they have been 6-bit (and fast) for many years. TN panels could just as well be 8-bit panels if the manufacturers decided to make them so.
So, as TN panels move into more upscale sizes, like 22", it's possible manufacturers have used that more premium size and price to squeeze in better color.