These are intended for LCD display tests. A really good display is capable of displaying 256 different grey levels. The cheaper ones won't go further then 4.
The image below should display 256 different grey levels, free of dithering (with a true colour driver of course).
The image below should display 16 horizontal bars of each 16 squares,
each successive square being brighter then the previous one.
With this picture you should be able to count the total number of grey levels;
If there are only 4 rectangles rather then 16 squares in each horizontal bar,
your display has 64 grey levels. If all the horizontal bars have a uniform
colour, only 16 grey levels are supported.
Some TFT screens will perform better with an analog VGA than with a digital DVI input.
The total number of colours is of course the number of grey levels to the 3rd power;
For a reasonable B&W picture quality, a minimum of 32 grey levels (15 bits) is required.