If you are using the Comcast.net webmail page, you cannot change the background color of the compose email screen. However, if you are using an email program such as Outlook Express for your Comcast email, you can edit the background color by starting a new message and pulling down the Format menu from the top of the new message, then select the Background option and you will see the color option to allow you to change this color.
If you are not using Outlook Express for your email and you want to try it out, take a look at this FAQ for instructions on how to set it up to access your Comcast email account. Also, if you are setting up Outlook Express for the first time, you may want to follow the instructions in this FAQ as soon as you finish the first one linked above in my post. This second one will ensure that your messages will still be accessible through the Comcast.net webmail page in case you decide you don't want to use Outlook Express after trying it out.