Colour Spaces

If I colour grade a film, should I colour grade in Rec. 709 colour space. It’s going to be shown at a cinema for one night so in that case should I grade for the P3 colour space?

Do most people grade in P3 than have it converted to Rec. 709 or vice versa?

I know a typical workflow has been grading with timeline and monitor Rec.709 then do a DCI export process.

If your monitor is really correctly calibrated for full P3 you might just do that. But more of the people i know use the Rec.709 grade trimmed to the appropriate delivery for that theatrical requirement.

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That works to some degree, but serious grading for cinema needs a P3 calibrated projector.
The viewing environment in a theater is quite different from looking at a monitor.

Yes you "should" have a proper p3 projector. Hover, you can get it within a very good approximation if you have proper rec709 monitoring, color for that and then do the appropriate rec709/2.4 to xyz/2.6 tiff export.

And yes, more than one of the movie i finished where done with this workflow in mind for multiple reasons.

I'm a fan of the ACES work flow, so my preference is to grade in the largest color space I will need, and then change the ODT for lesser deliverables.

That said, my work is not very high end, and has never been shown in a theater, so...caveat emptor.