Different colors only when viewing on iPhone

Hi Friends!

When testing my new website on multiple devices I noticed that its on one of my iPhones (XS Max) the content text is showing greyed out. Text should be black but on this device its shows like the text ist transparency to 80% or so. It also shows a black bg in cards when the original bg color is light grey. When testing on desktop on another iPhone it looks as it should.

Can someone help to make sure the results are consistent across all devices? And is there a way or a tool that you use to test and troubleshoot across multiple devices?

when testing on my laptop and adjusting the browser window it doesn’t bring the same results as when testing on a mobile device directly.

Thank y’all in advance.