You might need to use 2 sheets and a dashboard to do this. One sheet will represent your headers only, the other one will have the actual measure values. Both will have the 'Show Header' option disabled and will be then put together on a dashboard. An example is attached.
While it's possible, I often try to avoid doing something like this unless having different colors on the headers brings any value to the analysis.
This blog post has some great tips and tricks about formatting: Older But Still Useful – Conditional Formatting | Drawing with Numbers
Hope this helps.
Headers.twbx 1.2 MB