Direct Transformation - Absolute or Relative

Here is an example of a picture where the relative hues, saturations and luminances have all been adjusted (see Absolute v Relative Substitution):

You can make your transformations on the whole picture, since the surround is generally dark and does not show the colour change. (See All At Once tool.) The window at the side of the car is slightly affected, but this does not seem to matter.

There are many pictures where even if you change the background colour, it does not really matter. In the above case, you cannot see the difference, since changing the hue of black still gives you black. But in the next section (Well-Defined Objects) you will see a picture of a blue propelling pencil with a very light background. If you decided to process the whole picture with red, the background would come out as a very light pink. Whether or not this had any importance would depend entirely on what you wanted to do with the picture.