Colors changing depending on brightness

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.

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Expressing or conveying visual experiences effectively through these forums may be challenging. The underlying visual issue on your iPhone cannot be accurately diagnosed online. Factors such as your iPhone's specific settings, visual defects, and potential hardware problems cannot be assessed through this method. For more personalized and in-depth assistance, I recommend securing a Genius Bar Reservation and exploring the various support options provided by Apple.

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When you encounter issues with your Apple devices, the Genius Bar at Apple Retail Stores is the go-to place for expert assistance. The Genius Bar is staffed by knowledgeable Apple employees, often referred to as "Geniuses," who can diagnose and resolve a wide range of hardware and software-related problems. To ensure personalized attention and a convenient visit, Apple provides the option to schedule a Genius Bar Reservation.

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Hello, from what info i can gather, you may have a setting called "True Tone" enabled. You can disable this by going Settings > Display & Brightness scroll down untill you find True Tone, Hope this helps!

That's completely normal physics. You can see this on literally any screen, whether it's your smartphone, tablet, TV or computer monitor.

It's also why when you properly calibrate and profile a device, you cannot move any of the calibration controls (brightness, contrast, etc.) afterwards, or you invalidate the profile.

It doesn't matter what hue you're referring to, adding brightness will always result in a more saturated hue than what you started with, and lowering the brightness will reduce the saturation.