Here's some more food for thought: It's totally possible that some percentage of iPhone 6/Pluses have screens that behave like the OPs, while some don't. And yet both types are considered normal and not defective.
Why? Because not all iPhones have exactly the same components.
All the key iPhone parts (CPU, RAM, LCD) have more than one supplier. The A8 chip is rumored to be 60% made by TSMC, 40% by Samsung. The LCD could be similar: Maybe Samsung or LG is making some, while Sharp is making the rest.
This has been true through all iPhone generations, and even beyond just the iPhone. Macs, PCs, other smartphones, and any other device where millions of them have to be made end up like this. Even cars and refrigerators and other factory made things have multiple vendors for each part. It's just something that makes sense to do, so if one supplier folds on you, you have a backup plan.
Now, all of these parts have to meet certain specifications. But obscure things like "whether the LCD looks blank for a split second after being powered off for more than 5 minutes" might not exactly be on the spec sheet. So, maybe LCDs from one supplier look white for a fraction of a second when this happens, while the other supplier's LCDs don't do it.
I'm actually surprised that the annual "my iPhone looks more blue/yellow/red than my friend's iPhone" hasn't reared its ugly head yet. Maybe people are too busy bending their phones to notice any display color variations this time around. (EDIT:
Oops, nevermind.)