It was artistic stereotype. They could have used all-red Albatros as well. Historically it would have have been more accurate to use Fokker EIIIs and IVs but these would have looked too archaic beside the Nieuports and made it seem as if the Flyboys were shooting down guys flying "Penelope Pitstop"-type planes; not heroic enough. Same reason why
Lanoe Hawker is always shown flying an SE5a in movies about Richthofen and why the British bombers in "The Red Baron" were Handley Pages rather than FE2s.
Making the enemy planes all the same color made the action easy to follow and lessened the CGI developement time and bill.