How do I stop Preview from automatically brightening my images?

Apple-Oh wrote:

I frequently use Preview for editing images, including editing the color and brightness settings.

Don't do that.

How can I do this accurately if Preview isn't showing me the actual image

Easy answer - you can't.

(This is all different from "true tone" and other system-wide display settings - it's specific to Preview, and happens whether True Tone is on or off.)

It's a known problem. Preview doesn't properly handle colorspaces. See macOS Sequoia Bug - Preview misinterprets… - Apple Community

You'll have to use a 3rd party tool if you want accurate colour. I recommend Pixelmator Pro. Apple liked the product so much they bought the company.

I also have not seen or heard of this in any version of Sequoia.

The information that you provided regarding screenshots is relevant. This suggests that the system is "drawing" the image correctly, but something in how it is displayed in going awry.

You failed to mention something very important: what is your mac model, and what is the screen where you are experiencing this?

It seems to me that there may be something related to how HDR content is being displayed. Is your screen capable of HDR? (If it is the built-in screen in a recent M-series mac laptop, then it is)

Apple-Oh wrote:

Yeah, it's still happening in 15.2.

I doubt it's a bug, but as a new feature I want to turn it off!

I thought I said above but if not — I do not see the issue running any of the macOS 15.0—15.2

To trouble shoot further you can:

 —A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled temporarily.

Login and test. Reboot  as normal and test.  Caches get rebuilt automatically.

This test will tell you if third party interference; most extensions etc are not loaded in safe boot mode. 

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support 

 This will tell you if it a universal issue or isolated to your user/admin account. 

***if you have a wireless keyboard with rechargeable batteries connect the charging cable before booting to Safe Mode.  

       This will make it act as a wired keyboard and a successful Safe Boot mode. 

unplug all non-essential  peripherals when testing 

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS

Apple-Oh wrote:

Since the release of Sequoia, I notice that Preview is automatically brightening the current foreground image. Preview opens the image and shows it at its normal color settings, then turns up the brightness over about a half second.

never heard this before...

verify your software is up to date…

The current stable release of Sequoia including bug fixes, security updates is macOS 15.2

To be proactive you can file a bug report / submit your Apple Feedback here: Product Feedback - Apple

Is it all images or just images with a lot of text? If it's the latter then it could be Preview going through its optical character reading to allow you to find text.

I am having a similar problem. I copy a segment of an image and paste it into a new document. I then watch as Preview brightens it up... a lot. If I then copy that brightened image and paste it into Photoshop, it comes up completely washed out. I often swap between the two applications depending on my needs, and now I can't do that anymore.

There has to be a way to turn off this feature for advanced users.

Apple-Oh wrote:

Yeah, it's still happening in 15.2.

FWIW, I'm not seeing the issue. Preview on the left, Acorn on the right. Sequoia 15.2. I tried to size them equally.