Hi, congrats on your soon to be new puppy. How exciting!
I have used whitening shampoo on both Leroy and Evie and I don't find that their other colors look dull.
However, for Evie I have yet to find a whitening shampoo that works to get rid of her yellow stained coat. Unfortunately, when I got her the white part of her coat was stained yellow and it's gotten better but still stained. For Leroy, I use a whitening shampoo once in awhile and I mostly use the CC Spectrum 10 alternating with puppy shampoo and the white part of his coat is white.