Color of poop..more yellow then brown.

I have read all over the net and confused. Some say yellow to brown is normal. While other things say it means rapid transit.  Zane(8 months old) was a stray who had trouble with runny poops after we took him in.  It wasnt right away and was after they wormed him and gave him his first shots. Took the stool in and they think its his food, as the stool came back negative for anything.  Did the chicken/pumpkin thing for a bit. Got to sample packages of grain free dry. He is doing good, now with firm stool.   Bought a large bag and switched out the chicken for sheba chicken cuts. 

Stool still formed, but soft.  Sometimes it is kinda yellow. Not the darker brown like my other cats.  Is that normal? Or is darker better? 

He is an appointment tomorrow so I got a sample to take with. But its driving me crazy wondering if its normal or there is something wrong with him. 

Color can definitely have to do with what they are eating.  If it is yellow and not formed correctly (in other words runny and/or mucousy (sp?) and/or horribly stinky, that would probably indicate rapid transit.  But chicken is a light colored protein, so if he is eating that exclusively, I would expect his stools to be light in color.  Conversely, if he were eating only beef, I would expect them to be dark brown.