Yes, with kibble, sometimes a bag does seem to be different in color. And yes, dogs with sensitive systems will often show an issue with that. I don't know if it has to do with the environmental conditions the bags were kept in, or if the air hit the kibble somehow. Better dog food brands can be better at the quality bag to bag. If you have a question, take it back up to the store and get a replacement.
Jenna, Babs & Heidi
Odessa, Lassie & Mufasa
Milla & Ninja
Joy, Dolly, Bear, Karma & Cujo II