I wanted to start from scratch with my home networking.
I want to reset the ipfire completely. Set new IPs for Red/Green/Blue, delete all rules, addons etc.
Is there any way to do that easily?
Or should I just reflash the image and install again?
I doubt that you could reset reliably. In particular, successfully changing Green IP is fairly complicated.
It would probably be much quicker, as well as more reliable, to reflash the image. The backup file is simply text and you might want to retain that, as a reference for any rules etc that you do want to repeat.
Yes, I would recommend to reinstall the whole system.
You will benefit from some improvements to the partition layout and so on.