Sending text-based e-mails is easy, but making them fancy with colors, background, tables or other HTML tricks is another story altogether. Not all e-mail programs will accept all HTML-generated messages.
Some of our clients sidestep some requirements by subscribing to services that do this for them by providing templates. While some of these can help you create fancy e-mails, they're not perfect. Spam catchers often restrict messages that are sent from Constant Contact, for instance.
But you can still send custom, fancy HTML e-mails without using someone else's template with Premailer. This service cleans up your existing HTML file and puts it into a format that most e-mail programs can use, or at least alerts you to red flags so you can clean up your files yourself.
Give it a shot; this site has a sample set up so you can see how it works.