Final Cut Pro X: Work in Wide color gamut
Standard
Sets the working color space to linear RGB with Rec709 color primaries, the color space that previous versions of Final Cut Pro have used. Rec709 is the current standard for HDTV projects, including HD cable, Blu-ray discs, and streaming video. If you are outputting video for YouTube, Vimeo or playback over the internet, this is your best choice.
Wide Gamut
Sets the working color space to linear RGB with Rec. 2020 color primaries, a color space with a wider range of colors than Rec709. Rec2020 is useful for 4K and 8K UHDTV and HDR projects. If you are outputting video for screens or displays capable of displaying High Dynamic Range (HDR) colors, this is your best choice.
For general purpose video being watched over the internet (sRGB) you don’t need to use Wide Gamut as the displays are not capable of displaying the ‘whiter whites’ and highlighted areas such a clouds, skys, lights, etc. can appear completely blown out meaning there is just a patch of white and no detail in the image.
Working with both in FCPX
What if I have footage that was captured in Rec709 and Wide Gamut?
A lot of new cameras have the option to record in HDR, Wide Gamut, Cinema Gamut as well as traditional Rec709. In this instance, aim to be consistent in the color space you use and not switch between the two to save yourself time matching the look of clips in the timeline. If you have clips shot at different times, on different cameras with different color profiles you’ll need to look at conforming the clips, here’s some info on doing just that.