By default when you use a brand board color in the Email Builder it renders in the email code as an 8 digit HTML color code with opacity (e.g. #FF000080 or #FFFFFFF). Outlook doesn't handle opacity/ only handles 6 digit HTML colors.

This means that any time you use the brand boards colours in an email these colors are displayed as incorrect or blank.

I have been able to replicate this across multiple Email Builder items (e.g. buttons, dividers, etc).