First highlight your sub image then click on this icon:
Alternatively right click on the sub image and choose the item from the pop up menu.
Here is an example of when you might like to use this image:
Just by nudging up the contrast, saturation, and brightness you can transform the image into...
The controls of the dialog itself are divided into two methods of correcting photographs:
Top right shows HSV (Hue Saturation Value) and CMYK (Cyan Magenta Yellow Black). If the basic colors are right, but they are lacking in "strength" then use the HSV method. See lesson 4 for an example of using this color correction method.
To use the CMYK sliders (bottom right in the dialog) you need to know about the printing process, which is beyond the scope of this help file, though there are plenty of sources on the internet.