Hi Xascha,
Thanks for contacting Apple Support Communities!
We understand that your photos taken with your iPhone don't appear to retain the colors as you'd expect when you use the same photos on other devices, correct?
Do you happen to have Optimize Storage enabled for your iCloud Photos on your iPhone? If you have Optimize Storage enabled in iCloud Photos, smaller space saving versions of the photos are stored on your iPhone while the full resolution photos are stored in iCloud. You can download the photos from iCloud directly via a web browser on your computer to help determine if this is the cause. You can learn more about how iCloud Photos works here: Set up and use iCloud Photos - Apple Support
"If you need to save space on your device
iCloud Photos keeps all your photos and videos in their original, high-resolution version. To save storage space on your device, you can turn on Optimize Storage.
If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them."
We hope this helps.
Cheers!