In late 1916 the recommendation to mitigate the manpower crisis in Britain was to extend the age of conscription up to the age of 50 - comedy gold!
This step had already been taken in France, Austria and Germany.
The idea that the Entente running out of manpower is simply wrong. Finding 500,000 men to replace the Americans could be found by:
1. Extending the draft age in Britain to 50 110,000 men
2. Reducing the troops in Britain and Ireland 210,000 men
3. Culling the Navy, Mines and factories 270,000 men
(figures taken from Winston Churchill's memo of December 18, 1917
Colonial manpower did not have to be used in a combat role. They can be used in many rear functions and factories without any risk of future problems in the colonies. Even if there were real fear that increased colonial troops might cause problems in the future, they would still be used. After all, if the war is lost, India is lost