It is probably the chicken in the artemis. I fed artemis 5 1/2 years ago so most of their foods are chicken based at least back then. It could be ibd as well and ibd dogs do not do well on chicken per internal medicine specialist. You may want to try a venison and sweet potato type diet and see if that helps. Also I use probiotics in my one dog with allergies and it does help ibd dogs as well and is safe. I just got a pm from someone I helped on here yesterday that I helped a while ago and she had taken her dog to vet and once off antibiotics diarrhea came back. She switched off chicken food and diarhea stopped and she was so happy and thanked me for mentioning it. It is worth a shot but i would strongly advise the probiotics as well. Colitis can be tricky as sometimes it can be hard to get rid of. A dog can eat a food for many years and get an intolerance to it as well.
Stress can also cause colitis so is there any major changes going on before changing over food?
here is info
http://www.petplace.com/dogs/colitis-in-dogs/page1.aspx
It can also be triggered by a medication such as nsaid as well so have any meds been given?