Please show your flow and better explain what you are trying to accomplish. Right now, there is not enough detail and lots of guessing needs to be done on what you're trying to accomplish. For example, are you using a select action, converting it to an HTML table, and then inserting the table into an Outlook e-mail. If so, put the e-mail in HTML mode and add a CSS section at the top of the e-mail body.
Here is some very simple CSS to make the header yellow:
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