The correct answer to this open question is the following.
You did not specify what war you are talking about. Trying to help you we are going to assume that you are talking about World War I.
In addition to fighting in the army, European colonies contributed to the war effort in that these colonies sent supplies and food to the war front in Europe.
European superpowers had colonies in Africa, India, and Asia, places where there were plenty of natural resources and raw materials that were much needed by European countries during World War I. Great Britain and France were the European countries that used most soldiers from their colonies after many white troops were killed on the battlefield.