Captions turn grey with Wide gamut hdr

Apply a color correction. The simplest is the Color Board. Simply increase the overall exposure.

Have you tried selecting "Compositing" in the Inspector and fiddling with the Blend Mode?

Or select "Face" in the Text Inspector and change the colour.

Also try clicking on the Colour Inspector and manipulate the controls in that.

All titles are too dark in HDR. Titles can be controlled, unfortunately captions can not. Apple needs to code Text so it’s brighter in HDR projects.

Something that may or may not be any use is altering the appearancence of captions and subtitles in System Preferences/Accessibility/Captions

Have you checked whether it does it with a non-HDR clip?

There could be a glitch in FCPX . . . try deleting the preferences.

I haven't got an HDR clip to test so I reduced the contrast in a normal clip but it had no effect (as you would expect) on the whiteness of the captions . . . or has it?

Open the Colour Board and raise the exposure.

User feedback to report the problem.

When exported it does look a little off-white or is it my failing eyesight?

They both seem to be lacking brilliance.

Try using the "Snow Pencil" colour . . . is that a bit whiter?

Are you talking about captions, or titles as Ian assumed you meant?