I have an iPhone XR running iOS 15.4.1 and I’ve found that if I select dark mode (night mode) as my default display mode, when I go to any site that displays pictures / photos they look like negatives. If I turn off dark mode the pictures / photos appear as normal. So something in the dark mode selection causes the pictures / photos to change their appearance to that of negatives.
I sometimes get that effect, for images that are created by taking a screenshot - when using Private Browsing Only. Brave Browser for iOS v1.37, and Display Appearance is set to: Light.
The image appears OK on the iPhone, but when transfered to the Mac computer, the image appears as if a Negative Photo. The colors are inverted.