Why does my phone take only black and white photos?

There is a possibility that you changed a setting or filter to only black and white. Find where you set filters, and change it back to normal. If this doesn't work, then possibly your camera got damaged. As always, try restarting your phone as the first option. If you need more details on how to change filters, I can try my best to guide you through it. PS. I don't own a Xiaomi phone, nor any other Android phone, so my instructions may not be too clear, but like I said, I can try my best to help you.