Out of all the major power involved, it is my understanding, that they were the only one to use African soldiers on the western front. I am aware of a certain amount of African Americans fighting. Was it purely due to lack of manpower in France or was it a different attitude to their colonies or something different entirely?
The British used African troops too, just not many. Both Britain and France drew upon their colonies for manpower, and France had Senegal, Algeria, Morocco, Tunisia and Madagascar. Not all were soldiers, a great many were labourers (e.g. the British African Native Labour Corps) but there was the South African Brigade which saw action at the Somme, Arras and the Third Battle of Ypres.
Britain's other colonies contributed black troops, including men from the West Indies, but there was political resistance to using them, and they were not recruited to the same level the Indians were (the Indian Army contributed more than 1.5 million men). Outside of Europe, the majority of Africans used were either as labourers or bearers in the logistics chain.
Racial hierarchies were still popular, but the French have always seemed to prefer to assimilate external populations, whereas the British approach was one of separation, until losses forced the deployment of other races to Europe (Indians, Gurkhas, Sikhs). Despite a vocal contingent lobbying for more black colonial troops to be raised, the prevailing majority believed it was setting a dangerous precedent to let black troops fight white men in Europe, even when those white men were the enemy.