A few months ago I was working on a deliverability problem with a new client. They are a social networking site heavily branded with black background and pink text. One of the questions they asked me during the course of troubleshooting their delivery problems was if sending pink and black branded emails to match their site branding would decrease their deliverability.
That was actually a more interesting question than many I have received and led to the following analogy. A website is your showroom on the web. It is the equivalent of a brick and mortar store where people visit you and come to see what you have to offer for sale. Heavily branding the store is the right thing to do.
An email, be it marketing, transactional or relationship, is the equivalent of sending a traveling salesperson to someone’s house. That sales person is entering the customer’s space. In this case overly branding your presence in the customer’s space which can annoy or completely turn off your customers.
Branding emails to customers is a good thing; it builds brand recognition and customer relationships. Just remember, though, that you’re entering the customer’s space. Be respectful of that space.
As an aside, I did actually ask AOL about the color of email would decrease delivery. The nice folks over there did reply “AOL SAYS NO PINK!” But I’m fairly sure they weren’t serious.