Why does the same image look different on two screens?
sRGB, P3, and why saturated colours shift
sRGB is the safe common denominator that virtually everything can display. Display P3 is wider, covering more intense reds and greens. An image saved in P3 and shown on an sRGB screen has its most saturated colours flattened, because those colours simply do not exist on that panel.
For anything going on the web, export in sRGB. It is the version everyone will actually see correctly.
What you can fix and what you cannot
You can fix profile mismatches by exporting correctly, and you can reduce brightness differences by setting screens to a similar level in similar light. You cannot make a low-gamut panel show colours it does not have, and you cannot control what screen your audience uses.
The practical response is to design so the meaning does not depend on precise colour: adequate contrast, and never colour alone to distinguish two things.
Night modes and automatic warmth
Blue-light shifts and auto-brightness change the colour on the fly, which makes any judgement unreliable. Turn them off before assessing colour, and check in daylight — an image graded at midnight under a warm screen filter almost always looks too blue the next morning.
