The juice is healing for the whole digestive system and especially the collard or Kale green juice which also is great for the liver. If you persist with it continually it can only do you good. I usually have a pint a day now in addition to regular meals. You can add other juices like lettuce or cucumber instead of the Celery if you like and the apple is just for sweetness so you don't want too many apples.
Inflammatory foods are anything that worsens an already inflamed digestive system and that would include hot spicy food, red meat like beef and lamb, acid stuff like vinegar or citrus and also Sugar promotes inflammation.
Junk food in general is inflammatory, especially food fried in vegetable oil. The best oils are coconut, butter oil or ghee, and hemp or flax oil are ok since they are high in omega 3 but those should be used unheated. I would go very easy on oil in general till your digestion improves. If you take vitamin D though some fat or oil is needed to help absorb it.
Eggs are a good source of protein. Sunflower seeds are also very helpful for colitis if ground up fine. Almond milk is good too if you can make it. I use the strained pulp too
Proctitis often precedes colitis and it did in my case so you need to stop it before it spreads higher.
Sticking to the juice initially will give the best results, you could then introduce more soft fibre like cooked veg and smoothies made from not too fibrous things like bananas and spinach, then add the green juice to that.
Also the vitamin D is very importent for liver health so that's another reason to get your levels checked