Ever wonder how people create vibrant looking pictures and scenes? The mixing of pleasing colour combinations is one aspect of good artistry. In digitally reproduced images, there is another aspect.
Our eyes can perceive scenes in a human form of high definition. But when our eyes look at digitally rendered scenes, non colour corrected scenes can appear flat and un-appealing.
To overcome the limitations of low resolution devices, techniques have been developed to trick the eye (and associated visual interpretations) to perceiving scene pleasantness. Much science has gone into understanding the why's and wherefore's of this. One starting site with links to a wealth of information is Cinematic Color.
The lead off paper on that site, Cinematic Color: From Your Monitor to the Big Screen provides a good over view. Buried in the middle of that paper is a innocuous chart with a single curve on it. The curve is commonly referred to as the S-curve of colour grading.
In essence, you want to make sure that there are lots of details in the darker areas, and lots of detail in the bright areas. (blowing out the high-lights is a no-no, but even this rule is can be broken in artistically pleasing ways) Anyway, this curve sacrifices the mid-tones. But the sacrifice seems to make all things right.