You need to send a test to your colleague straight from your email marketing section in the same way you did for yourself except in the pop up window add your colleagues email address, not yours.
Just click on the 'Send Test' icon as shown below:
Enter your colleagues email address. This will ensure that they see the email template correctly.
Forwarding emails on that you have received in from a test email will strip down the format - this is done by your email client i.e. Outlook, Gmail etc - something we have no control over.