yellowish fur on a white cat:any ideas how to clean it?

[tommy & hazel]

hi guys, im new here

i have this wonderful 5 months old long haired white and black cat by the name of Tommy of which i truly love. its just i have this one nagging problem where the underbelly coat looks yellowish despite numerous attempts to clean it including using whitening shampoo as per recommended by my petshop guy.the rest of the coat looks white all right,only the underbelly part seemed to be problematic.

i wonder if there's anyone with similar experience or any idea that might be of help here

many thanks!