You'll need to edit the template HTML.
To access the HTML, you go to the Design Studio -> Emails -> Templates -> Amber Hope (edit draft).
At this point, it's probably easiest to copy and paste and give the HTML code to someone who knows HTML.
If you like, you could try copying and pasting all the HTML into a notepad. For Amber Hope, you could do a find and replace of "fad00c" with "000000". This will make your colour change from yellow to black.
The good news is that I've heard Marketo is bringing in a new "guided template" feature for their emails, which will allow templates to be set up with "variables" that can be changed in a WYSIWG editor - or in English, you'll be able to change the colour of emails in a much easier manner in future.