It's is true that Westies' white hair somnetimes becomes yellowish. The solution is a blue shampoo. Be sure to check that your dog has no skin irritations before washing with blue shampoo and also be sure to rinse with warm water A LOT after.
Just use this blue dog shampoo now and then; for the regular every 3 weeks time bath use sensitive skin shampoo and always dry your Westie with warm air, not hot.
It's easy to start a Westie into skin problems that can become cronic, be careful.
Just a note to remenber that some Westies have a weathen shade in their undercoat in the back. This is normal and will never become white.
Good luck and good bathing.
--- --- --- --- --- --- --- --- ---
Before all, congratulations for the fantastic site!
The hair of a Westie called Lumi is getting a little yellow, in spite of the baths and the scrubing.