Colors look different after export
You may need to perform a “trim pass” to export to Rec709 colour space.

See: https://images.apple.com/final-cut-pro/docs/Wide_Color_Gamut.pdf

Matching colors in Rec. 2020 and Rec. 709 masters

When some colors are translated to the narrower gamut of Rec. 709, they may look
different. To ensure that the Rec. 2020 master and Rec. 709 submaster look similar, you can perform a trim pass color adjustment on the Rec. 709 timeline to bring all the colors in line with broadcast specifications. When performing a trim pass in a Rec. 709 timeline, you should attempt to match the look and feel of the Rec. 2020 timeline as closely as possible.

Prepare for the trim pass


In the Final Cut Pro browser, select a master Rec. 2020 project and then choose
Edit > Duplicate Project as Snapshot.
With the duplicated project selected in the browser, choose Window >
Project Properties (or press Command-J), then click the Modify button near
the top-right corner of the inspector.





In the dialog that appears, click the Color Space pop-up menu and choose
Standard - Rec. 709.
In the browser, click the name of the duplicated project (the submaster), then type
“Rec. 709” at the end of the name.
In the browser, click the name of the original project (the master), type “Rec. 2020”
at the end of the name, then press Return.