user2538: Colors look great in FCPX.
After I export (Share > Master file > H.264 Quicktime) they look a bit dull: desaturated, less vibrant. Also sort of like the wrong gamma curve - shadows are darker.
I realise compression is lossy but is this expected?  Should I over-saturate the video in FCPX a bit to compensate?
I saw some very technical forum posts talking about color-space conversions last time I googled for this, suggesting maybe Quicktime uses the wrong 'matrix'.  I am not a video pro but the visual difference does remind me of what can happen with RGB > CMYK and colour profiles in the print world. 
At first I thought it was only after upload to YouTube, but the issue is there when I play back the exported .mov too.
Does anyone have any tips or info?

user21348: 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.


user33077: I have this issue forever...found a solution thanks to this website
https://isaac-molina.com/downloads/
download his luts aplly the correct lut to an adjustment layer placed on top of you project (in my case convert mrec 709 g2.4 to sRGB g2.2) and then you get to see what you get as output, but inside the project viewer, so you can grade correctly.
Good luck

2538: <p>Colors look great in FCPX.</p>

<p>After I export (Share > Master file > H.264 Quicktime) they look a bit dull: desaturated, less vibrant. Also sort of like the wrong gamma curve - shadows are darker.</p>

<p>I realise compression is lossy but is this expected?  Should I over-saturate the video in FCPX a bit to compensate?</p>

<p>I saw some very technical forum posts talking about color-space conversions last time I googled for this, suggesting maybe Quicktime uses the wrong 'matrix'.  I am not a video pro but the visual difference does remind me of what can happen with RGB > CMYK and colour profiles in the print world. </p>

<p>At first I thought it was only after upload to YouTube, but the issue is there when I play back the exported .mov too.</p>

<p>Does anyone have any tips or info?</p>
 21348: <p>You may need to perform a “trim pass” to export to Rec709 colour space.</p>

<p>See: <a href="https://images.apple.com/final-cut-pro/docs/Wide_Color_Gamut.pdf" rel="nofollow noreferrer">https://images.apple.com/final-cut-pro/docs/Wide_Color_Gamut.pdf</a></p>

<p><strong>Matching colors in Rec. 2020 and Rec. 709 masters</strong></p>

<p>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.</p>

<p><strong>Prepare for the trim pass</strong></p>

<ol>
<li>In the Final Cut Pro browser, select a master Rec. 2020 project and then choose
Edit > Duplicate Project as Snapshot.</li>
<li>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.</li>
</ol>

<p><a href="https://i.stack.imgur.com/W81sF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/W81sF.png" alt="enter image description here"></a></p>

<ol start="3">
<li>In the dialog that appears, click the Color Space pop-up menu and choose
Standard - Rec. 709.</li>
<li>In the browser, click the name of the duplicated project (the submaster), then type
“Rec. 709” at the end of the name.</li>
<li>In the browser, click the name of the original project (the master), type “Rec. 2020”
at the end of the name, then press Return.</li>
</ol>
 33077: <p>I have this issue forever...found a solution thanks to this website
<a href="https://isaac-molina.com/downloads/" rel="nofollow noreferrer">https://isaac-molina.com/downloads/</a></p>
<p>download his luts aplly the correct lut to an adjustment layer placed on top of you project (in my case convert mrec 709 g2.4 to sRGB g2.2) and then you get to see what you get as output, but inside the project viewer, so you can grade correctly.</p>
<p>Good luck</p>
