You can't with a single in-app command, but there are a few photo presets available from DirectorZone that will do that. This one works with full color, and this one is negative B&W. Both were created in 2015 by PIX, the forum moderator.
youtube/optodata

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It can be done in PhotoDirector and PowerDirector. Optodata answered first. There is still another way to do it by reversing the rgb curve as shown in this tutorial: https://www.youtube.com/watch?v=IVda149y8O0 . Choose any method.

Thank you, optodata, for your kindly plug!

Here's an image showing how to "invert the colours", which is one of the methods shown in the tutorial tomasc linked. How lovely to see one's presets featured in a tutorial. Next step, Hollywood???

PIX PhD CDR ADR DZ MVL PP Forum Moderator
PIX YouTube channel

This is great that PIX was the one that came up with the solution 5 years ago in the reply to another user who asked. Have to say that it was not found in a forum search here for invert color or positive to negative. Do not know what the OP search terms were to find that 2013 post. Some work needs to be done on the forum search to make it better. Outside google search leads to the tutorial by a different user.

Link: https://forum.cyberlink.com/forum/posts/list/46722.page#post_box_242431 to PIX's solution. Thanks You PIX for the solution.

Hi tomasc

I found it with google - not this forum search. I tried several searches and nothing came up but google found it

I wil try the suggestions in this thread

thank you everyone!!

Thank you - this worked exactly as I needed!