No and it’s a very bad idea.
While I do understand that people do want to keep colors, this is currently out of Zammads scope and will be sanitized automatically. If you want to see colors, you can always download the RAW or HTML format to see original encodings.
Zammad itself does remove a lot of unwanted stylings to ensure the function of the UI.
Most mails come with body { color ... !important } directives (there are worse) which would nuke Zammads UI drawing it inaccessible. Trust me, I’ve seen it, you don’t want it.
Maybe it’s an idea to render the email message’s in an iframe. The contents could even be directly injected in the iframe via javascript if needed. This way the message won’t inherit the styling of the main page.