My I movie on my mac book pro changes the colour contrast as soon as uploaded what to do?

If you are importing photos from your cell phone taken with HDR, that can cause the photos to display very brightly in iMovie on Mac operating systems that have not yet updated for compatibility with HDR.

Try upgrading your OP and/or try removing the HDR and see if it makes a difference.

See the following link to a thread on this forum in case anything in there helps you:

When I import videos from the IPhone 12 I… - Apple Community

. . . and this link:

Adjust HDR camera settings on iPhone - Apple Support

If the above is not the problem, try deleting preferences. To delete preferences, open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can import without the undue brightness.

Also, you can try the following procedures:

Resetting the System Management Controller

Try resetting the System Management Controller (but no longer available for the new M1 computers):

Reset the SMC of your Mac - Apple Support

Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM,.:

Reset NVRAM on your Mac - Apple Support

-- Rich