This clip shows you a lot about colour grading and complementary colors.
Complementary colours are pairs of colours which, when combined, cancel each other out. This means that when combined, they produce a grey-scale colour like white or black. When placed next to each other, they create the strongest contrast for those particular two colours.
To see what the opposite colour is for, for example a skin tone, have a look at your colour wheel. Here you can find the colour wheel on the Adobe site.
Watch the movie and play around with a picture and try to create another colour grade.