Hello CindyLiaw.
Thank you for using Apple Support Communities. It sounds like you're experiencing issues when using your camera to take photos. Specifically, you're taking pictures of a projector and the background is suppose to be white but they're coming out in different colors. Allow us to investigate this issue further and and assist in any way we can.
To confirm, is this issue only occurring when taking photos of your projector or does it occur while taking pictures of other things? Which iOS version are you currently using? You can find your iOS version by going to Settings > General > About. We ask because if you have any pending updates available, it would be best to create a backup and update your device. Keeping your device fully updated can help resolve various issues and help with overall performance.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
If you're already fully updated, we can share some helpful resources that you can review. They contain troubleshooting steps and information on certain camera settings you can change to see if it helps:
If the camera or flash on your iPhone, iPad, or iPod touch isn't working - Apple Support
Change advanced camera settings on iPhone - Apple Support
About the Camera features on your iPhone - Apple Support
Feel free to let us know if these resources help out. Cheers.