How do I transfer a 60-second video from iMac to a colleague?

put it on a thumb drive or upload it to the cloud and share it unless you're on the same network and you can transfer it directly or via a file server of some kind. If they have a Mac or idevice in close proximity you could use air drop to send it to them.

You haven't provided enough information for us to be any more specific than that.

Share photos and videos on Mac - Apple Support describes the usual methods (basically, use the Share icon) but more information would help.

Telling us:

— the file size and the format of the video

— what devise your co-worker is using

might help us better help you.