What is the best export setting for YouTube?
MP4 with H.264 at 1080p or 4K inside the bitrate bands above, audio at minus fourteen LUFS integrated. The platform re-encodes everything, so the goal is arriving clean rather than mastering.
H.264 or H.265?
H.264 plays everywhere and uploads fine; H.265 exists for smaller files of the same look, at the cost of heavier decoding on older players. Uploads stay with H.264 unless file size is a real constraint.
What export should clients get for review?
A light H.264 at 1080p will do: small enough to send, sharp enough to judge the cut. The master is for delivery day, not for the review round.
Why do colours change after export?
Colour shifts almost always come from tags, not encoding: export in the same colour space the footage was tagged with, and verify once with a test render before a big delivery.