If Britain and France needed more men to continue the fight wouldnt they have been able to recruit more from there colonies
I am sure I have read somewhere that the Indian army in WWII was the largest purely volunteer army ever raised. Maybe a manpower desperate Britain could have offered independence in exchange for defeat of Germany.
Problem is that with both the French and British armies in the Great War, colonial forces were considered élites and suffered much higher casualties. For the French, the Senegalese and Morrocan tirailleurs were always in the first wave of attackers in a major offensive, and the Brits put the Canadian Corps in front of many of their offensive in Flanders and Picardie. The British didn't send Indian units to the Western Front because they were sending them to Mesopotamia and Palestine trying to knock the Turks out, or to East Africa to conquer German colonial forces under von Lettow-Vorbeck.
Anyway, the Great War ends in 1919 no matter what. Without the Americans coming in, the French are at the end of their rope in terms of manpower. They were so desperate that OTL Pétain was urging Clemenceau in mid-'18 to call up the class of 1921. 17 year olds! By the time of Operation Michael in March '18, the French were already dipping into the class of 1920.
The Germans aren't much better off either. Sure, they have more manpower, but they can't feed themselves. The homefront is collapsing, and soldiers on leave see this. They might be eating pretty well at the Front, but their families are living off of rotten potatoes and turnips. This isn't exactly news to inspire the "offensive spirit."