iphone 16 display appears gray

Make sure True Tone is off for both devices before you compare.

Settings > Display & Brightness > True Tone > Off

If they're still noticeably different, it’s probably because Apple subtly tweaks the color tuning with every new iPhone generation. Even if both devices use OLED.

They also source their displays from multiple manufacturers, with subtle, minuscule differences between them.

No two displays will ever be identical. Especially when comparing a new iPhone 16 to an iPhone 13 Mini, which was released in 2021, 4 years ago. The 13 Mini screen has many more hours of use on it than your 16. And honestly, from the photo you showed, I would be hard pressed to say there is a significant difference between the two. But, that's from looking at a photo and not seeing them side by side. If you think there is an issue with your iPhone 16, schedule an appointment at your Apple Store Genius Bar, but I'm very confident Apple won't find an issue with the screen.