Thanks for the multiple replies!
@helpful: I really appreciate your post! I like strong convictions and find your tone very convincing. Unfortunately, I don't entirely understand your advice: how comes that you change white balance in seconds? Or tenths of seconds, exposure settings? Do you manually adjust K on white balance? How exactly do you proceed?
I use a pure white target (white is really a reflection of an equal mixture of the three wavelengths of light) which is not pointed directly at the light source, but between a 45 to 90 degree angle. I try to duplicate the angle of someone's face relative to the light source. I can just use a white paper for simplicity. The side of the paper in the shade usually has a different color cast to it, so go with the side that is most representative of the lighting on your subjects (whether they are people in a room, people on a basketball court, etc.)
I make sure that the RGB histogram is enabled and change the exposure settings to properly expose the paper at a shutter speed 1/10th or below if I am indoors, except under incandescent lighting which does not have color shifts during the alternating current cycle. In natural light (e.g., daytime outdoors) or in 100% incandescent lighting, the shutter speed is irrelevant. I set the custom white balance to the paper, and check the RGB histogram to make sure all color channels are getting the same exposure. I repeat if necessary.
Adjusting Kelvin lets you change the receptivity to the wavelengths of light all at once, but it is not exact as taking a photo of a white subject. White contains an equal amount of all three color channels, red/blue/green, and so it allows all three color channels to be perfectly calibrated to the light source, with a single exposure.
I agree that most of the Av shots have strange exposures. How can I learn the correct exposure / aperture ? I'd be still relying on the Ettl metering to check manual settings. Well, I'll try harder to practice manual mode! Now I am tempted.
During the time when you were setting white balance, you can also determine the proper exposure value for the lighting conditions at the same time. If you know the brightness of the paper, you can use it in place of the neutral gray card (18% usually, but there are arguments about that.)
Your LCD can be a very bad idea of the exposure, especially if it is set to automatic brightness adjustment. (If you get your finger over the sensor below the LCD on the 5D3, for instance, the LCD will go very dim even though your picture's exposure is unchanged). But your histogram is an accurate indication of exposure.
You can look at the RGB histogram of the white paper shot (after setting the white balance) in order to set your exposure. (And this time, just use the same shutter speed and ISO that you expect to use for the action you plan to be shooting). Take a picture of the paper when it is directly facing the light source and you should see a thin high peak for each color channel. Change the exposure (I am assuming you are using manual exposure to be able to control aperture, shutter speed, and ISO) so that the peak for each color channel is about 10% below the right of the histogram. Your paper should be directly reflecting ("aimed at") the light source under the same lighting conditions that your subjects will be photographed in.
Note that there are often different types of light in big stadiums (i.e., some pinker lights, some bluer) and if you are at the wrong spot, sometimes only a row of blue lights are hitting the subjects, for example. And some lights in bad high schools are actually missing parts of the color spectrum. In those cases, proper colors are just about impossible, but it doesn't have anything to do with what format of image you are recording.
I am not trying to bash RAWs. They are a good way to save all the camera's information that was used when the shot was recorded and processed to JPEG, just in case any of that is needed. I will "turn them on" whenever I feel like it. But they only give 6 more bits of data, and a tiny exposure error (just less than one stop) will neutralize any benefit that the RAW may provide.
The reason why I started this thread is that with this camera just like other posters found my jpg look just like postprocessed RAWs already ! I use Aperture plus NIK suite. Could not sharpen better, could not impove the colours, could not significantly improve contrast, so I started putting RAW on CF and jpg on SD but ended up scratching my head. Well, try yourself. It's still early days with the mark III, and I don't state that it's only jpg worth, just that the jpg machine is good and tweaking the incamera settings on contrast, sharpness etc yields a good time saving for non-critical work.