saving Preview annotations color

If preview is corrupted : Enter in user library ( ~/Library ) please use the path > Mac HD > users > Home folder > library > containers > com.apple.preview > data > library > preferences > right click on com.apple.preview.plist and move to trash . Restart the computer & empty the trash .

As a guess, if the jpg is actually a monochrome image I don't see how Preview could save color info to it.

You can't do the conversion in Preview. You would need a graphics program such as Photoshop, GraphicConverter, Affinity Photo, etc. If you don't have one and would prefer a free program you could use GIMP or XnConvert.

FWIW, I took a monochrome/gray scale image and added a red arrow, which as I suspected, turned gray. I then converted the image to 32bit color, added a red arrow and now it saves as red.

In Preview -> Tools -> Show Inspector -> ℹ -> General

what is the "Color Model"

what is the "Depth"

I can guess. 😎

Your post has nothing to do with the problem. Preview cannot save color annotations to a gray scale image.

You have to convert any grey scale image to color before you can add color annotations. PDFs are not image files.