Does the image have a profile embedded, what is it, what are Photoshop’s Color Settings and is Windows Image Viewer color managed?

Please read up on Color Management. (At least in the Photoshop Help …)

Given that Photoshop and Windows Photo Viewer are both fully color-managed applications, something subtle may be going on here.

You don't have Photoshop set up in View - Proof do you?

If not, then it seems likely you have a color-management problem hinging on a badly formed monitor profile or possibly a display driver problem in which Photoshop or the Windows Photo Viewer are failing to perform proper color-management based on the image and monitor profiles.

Let's start with this, and hope the thread doesn't degrade into yet another slugfest about color-management...

What profile is associated with your monitor at the OS level?  Did you put it there via a calibration/profiling process?

-Noel

I suppose that's possible, Christoph.  I don't know what Windows Photo Viewer does with untagged images.  But given that Photoshop defaults to treating untagged images as sRGB, and Windows defaults to sRGB for monitors, it seems that the chances are that an untagged image would look the same.

But of course we can't get anywhere by guessing, hence the request for more information.

Let me add to my prior request:  What profile describes the document in Photoshop?  You can display that information in the little status box at the lower-left of the workspace, or in the Info panel (with appropriate settings in both cases).

-Noel

But given that Photoshop defaults to treating untagged images as sRGB

If sRGB is set as the RGB Working Space – which we won’t know for the OP’s setup until they finally provide some more information.

That happens too.

Another poster has said that Windows Photo Viewer, when in slide show mode (full screen) isn't color-managing his images the same as when the image is shown in a window.  I wonder if that could be factoring in here...

-Noel