Colour balance.

 If you are creating an image or a photomontage, it is important that when two or more images are put together, your final finished product looks realistic and the colour balance in the overall image is correct. 

A way of understanding this is that adding or removing certain colours can affect other colours drastically. A way of understanding this is refer to the colour balance wheel as a guide.

This information is handy to know when creating images for clients as it will help you understand colour in relation to other colours. 

                                           color balance wheel - Search (bing.com)