All of the colours that can be displayed on your screen derive from RGB (Red, Green and Blue). That is how the set works. Each pixel has a tiny little 'light' which is either red, green or blue. How they are illuminated creates all of the other colours.
This Wiki article goes into detail on it if you're interested:
http://en.wikipedia.org/wiki/RGB_color_model
Green is, in fact, a primary colour. It is an additive primary colour - used when different colours are added together to form other colours, as in a projector for example, or a plasma or LCD or CRT screen. You’re thinking of the artist's primaries of red, yellow and blue. There are various sets of primary colours - RGB, CMYK (in printing), RYB (paints etc). All that is meant by 'primary colours' is a set of colours from which all (or at least a great range) other colours can be derived.
Edit: I gave you a lot of information but am not sure I answered your question :) The slides are Red, Green and Blue because they are the only colours your screen can display. All the others are created from those three - so there is no 'yellow' sub-pixel in your screen. Yellow is created simply from the RGB set of little 'lights' in each pixel as described above. In fact, you might query why there is a white slide in the 'break-in' set. White is just all of RGB combines so arguably there's no purpose served by the white slides. The objective of the slides is to age each pixel by the same amount during the early life (200 hours typically) of the set. Pixels age very rapidly in the first 200 hours and then settle down so by using each pixel identically, they will all age at the same rate. This is important for image retention because what IR really is is a pixel or set of pixels that has aged at a different rate to the ones surrounding it. So if you keep a bright logo on screen, in one place, permanently, you can see that that area of the screen will have pixels that age faster than an area that has no such logo.
Yes - the brightness and contrast controls are misnamed. Brightness = black level and Contrast = white level. On the ST50 series you won't see any clipping when you advance contrast to the max. That's fairly common on modern-day sets. To set the brightness you can use instruments and software to achieve a given Foot-Lambert rating or use a test disc and set it by eye. Set it so that whites look white and the overall brightness in your room lighting conditions doesn't strain your eyes. In a dark room you obviously need less brightness than in a lit room for example.
Yes - contrast is your 'brightness' control. Just remember that Brightness = black level and Contrast = white level and it will all click into place. If you adjust 'Brightness" you adjust the black levels in your picture and the white level (theoretically at least) is not affected. If you adjust "Contrast" you adjust the white level and the black in the picture remains unaffected. So in reality in order to change what most people would call the 'brightness' of their picture, you need to adjust the 'contrast' control. In the ST50 there is no harm done to the picture by having contrast set at 100 - but the picture may be unbearably bright although it still won't be clipping the whites. Adjust the contrast to the level that looks good and is comfortable to your eyes - as I said earlier, you will need less 'brightness' in a dark room than in a lit room.
It would help a lot if manufacturers stopped using these silly names for the controls and simply renamed Brightness to Black Level and Contrast to White Level - names that actually describe what the controls do.
BTW, my comments about clipping the whites apply to EU sets - they do differ slightly to the North American equivalents, but i haven’t heard of any differences with regard to clipping white levels.
