An edit that looked correct on the timeline frequently plays back washed out or oversaturated after export. The pixel values are usually unchanged; what changed is how the player was told to interpret them.
Colour is carried as numbers plus instructions
A video file stores numbers describing each pixel, and separately stores tags saying which colour space and transfer curve those numbers belong to.
The same numbers mean different colours under different tags, so a file with the wrong tag displays a different image while containing identical data.
Editing software applies the tag it believes is correct, and a player elsewhere may apply a different assumption, which is where the shift appears.
Levels are the most common single fault
Video has historically used a limited range, reserving values at the extremes for signal headroom, while computer displays use the full range.
A file encoded as limited range and played as full range shows crushed blacks and clipped highlights. The reverse produces the flat, milky look editors recognise immediately.
Because the difference is a single flag, it survives every quality check that inspects the image on the machine that produced it.
Player behaviour is not consistent
Some players read colour tags carefully; others ignore them and assume a default. Browsers, operating system players and platform embeds all differ.
An export can therefore look correct in one application and wrong in another on the same computer, which makes the fault appear intermittent.
Checking an export in at least two unrelated players is the fastest way to distinguish a genuine grading error from a tagging one.
Uploading adds another interpretation
Platforms re-encode everything they receive, and that process reads the incoming tags and writes new ones for delivery.
Where the incoming tags are missing or contradictory, the platform guesses, and the guess is applied to every viewer rather than to one machine.
Wide colour material is the most exposed, because a platform may convert it for standard displays and the conversion is not always faithful.
Confirming the grade rather than trusting it
Scopes read the underlying numbers rather than the displayed image, so a waveform stays truthful even when the monitor is lying about the colour.
Setting the project's colour management explicitly, rather than relying on automatic detection from the first clip, removes the largest source of mismatch.
Where a grade must survive unknown players, staying within conventional range and avoiding extreme values leaves room for misinterpretation to do less damage.