Problem No. 1: How To Display Color 250
The simplest answer lies in making a pixel display the colors 248 and 252 alternately. If the panel is fast, your eye will only detect one color, the intermediate one between the two, i.e. 250. The right color displays in two movements.
The second, more accurate, method consists in working on four, rather than a single pixel. In this case, two of them display the color 248, two others the display a 252. The right color appears in one movement.
Problem No. 2: How To Display Color 249
In method 1, 248 is displayed twice, and 252 once. So it takes three movements this time to display a 249.
In method 2, an initial pixel displays 252, and the three others 248. The right color is displayed in only one movement.
What this means is that even if the first method is slower, at least it only involves a single pixel. The risk with the second, by using four pixels to only draw one, is that details of the image will be lost.
We haven't been let into the manufacturers' confidence and no doubt they have other methods that are less simplistic than those we have explained here, which may consist, for instance, of not always using the same pixel to display a color:
They may also decide to use not just four, but even nine, 16 or even more pixels, etc. Depending on the method adopted, the right color will be displayed fairly fast, you may experience a tiny flash and, and when the images follow each other in quick succession as in a DVD or in a game, the images may be more or less grainy. So between a desire for better color and a desire for a quicker response time, it's had to find a machine that will make it possible to assess the quality of an LCD screen.
Problem No.3, Why 16.7 Million Colors On The One Hand As Against 16.2 On The Other?
Quite simply because in 6-bit color, the maximum achievable value is 111111. This value, converted to 8 bits produces 11111100. The following values, 11111101, 11111110, 11111111 are thus inaccessible whatever dithering method is adopted. With dithering, the 16-bits do not actually display all 256 colors, but only 253 per RGB component, making 253 x 253 x 253 = 16.19 million colors. This is then rounded up to 16.2 million. So that's how you can distinguish an 18-bit panel from a 24-bit one. Only the latter will claim to display 16.7 million colors.
Thanks are due to Blue Apple, whose reports are pretty scathing, but who are passionate about the subject; they make enthralling reading, and it was they who raised our consciousness about this subject.