I sent several color photos with a neutral background and they were rejected. I would like to understand why.
With the exception of the second photo, focus is an issue. You need to work with a smaller arperture. Your background has a texture that appears grainy. I would perform noise reduction.
As mentioned the focus isn't very good, nor do I think the background is a good choice.
Have a read of this guide to get you started:
On the 2nd image, the only one that is in focus, I believe it was your intent to leave the center for the user to insert copy; however the texture of the background is mottled, and there are bits of debris which are visible when you zoom in. The bright purple doesn't seem to be a good choice either. Perhaps a neutral color would have been more acceptable.