After watching the new Wonder Woman movie I was reminded that the British Empire extended to India by a large number of soldiers and dignitaries walking in the background of London. Were there a ton of colonial troops from the UK and France in the Western European trench fights, or were colonial units mostly used to defend/attack the region they were raised in?
British and French policy on this differed but as a general matter, yes, WWI was an imperial war and there were an enormous number of colonial subjects serving within the armed forces on the Western Front, but also as non-combatant laborers on the Western Front, and also, as you suggest, as troops within their home regions.
Policy-wise, Africa was one of the major areas of difference between France and Britain. Whereas Francafrique troops could be and were called into service on the Western Front, the British did not use African troops outside the African theater (which was itself incredibly deadly and is an overlooked theater of WWI).
Both the British and the French made extensive use of East Asians as laborers on the Western Front, including both colonial subjects like the Vietnamese as well as hired Chinese labor.
The British use of Indian troops was extensive. They both served on the Western Front and also constituted the bulk of the Mesopotamian Campaign and eventually a very large portion of the Syrian campaign.
Colonial of course also included the white colonies, Canada, New Zealand and Australia. These troops also served both in the outlying theaters and on the Western Front.
Sourcewise: The Cambridge History of the First World War has good sections on this.