Well just looking at the ingredients is enough to send a shudder down my spine!
Corn meal, chicken by-product meal (a source of natural chondroitin sulfate and glucosamine), animal fat (preserved with mixed tocopherols and citric acid), soybean mill run, flaxseed, chicken liver flavor, dried egg product, dried chicken cartilage (a source of natural chondroitin sulfate and glucosamine), taurine, L-lysine, L-tryptophan, L-carnitine, DL-methionine, L-arginine, preserved with mixed tocopherols and citric acid, minerals (potassium chloride, salt, dicalcium phosphate, ferrous sulfate, zinc oxide, copper sulfate, manganous oxide, calcium iodate, sodium selenite), rosemary extract, beta-carotene, vitamins (choline chloride, vitamin A supplement, vitamin E supplement, vitamin D3 supplement, L-ascorbyl-2-polyphosphate (a source of vitamin C), niacin, thiamine mononitrate, calcium pantothenate, pyridoxine hydrochloride, riboflavin, folic acid, biotin, vitamin B12 supplement).
Corn meal, very hard for a dog to digest, chicken by-product meal

what's that all about! animal fat, how's that rendered and what animal is it coming from, soybean mill run

again not easy for a dog to digest and heat treated hulls normally fed to cattle, flaxseed, not needed and again a seed, chicken liver flavour

how do they make the flavour and what do they make it out of

dried egg product, well is it from egg or what sort of product, if it's dried egg then why don't they just say dried egg then

dried chicken cartilage

The rest are basically amino acids and preservatives which would all be found naturally in a raw diet.
Now all this has been produced, rendered and preserved and is totally artificial for a carnivore to eat so I'm not surprised the poor dog is suffering from colitis, she clearly cannot digest this properly and it's making her colon inflammed, as well as being very expensive 'food' I personally think they would be better off feeding something different and I'm going to naturally say feed her a raw diet as this is what I would do if she were mine. Labs are quite prone to digestive problems but all those that I know that used to suffer from colitis and that are now fed a raw diet no longer suffer from this. There could be an underlying reason and it could even be auto immune related.
I'm amazed the vet has continued to recommend this food especially as she is still suffering and there is no improvement. It will wear her down in the end and she'll start to deteriorate in other ways if they don't get her off this stuff.
Chicken intolerances when fed in it's raw natural state is quite rare but when there are chicken by-products and other chicken things in dried food then this often causes a problem due to the way it's produced so people automatically assume their dog has an intolerance to chicken. Why not suggest they switch to a raw diet or at least to another dried food such as Arden Grange which doesn't contain as many odd ingredients that Hills does.
Hope she gets the chance to make a turn round and gets some relief one way or another, keep us posted
Sandy