Here's what s/he said in reply:
"'Natural' baby shampoo is made up almost entirely of oil based products, which can prevent your brushes from picking up pigment and can even make them a little greasy. Regular baby shampoo, aside from the recent formaldehyde controversy that could lead to contact dermatitis, people don't realize that baby shampoo is not only much more abrasive than the label suggests - but also that it does nothing to preserve the COLOR of your brush hair.
Shampoo for color-treated hair is fantastic. I use Pureology because it doesn't have sulfates and a lot of really god-awful ingredients. Nothing is perfect, but I do try to be conscious of what I'm using. A $7 travel bottle of Pureology lasts a few months. I have brushes that have been around since I started doing makeup almost 10 years ago!
Also when I can score free products, I actually do like the Bobbi Brown brush shampoo as well. Not something I'd pay a fortune for, but a great product for those of you that are wondering.
Hope that helps