Why is 'white' in Photoshop FFFCDD?

That looks like a defective monitor profile in the operating system. You can check two things though :

1. In Windows , go to Settings > Display and make sure HDR is off

2. In Windows type 'Color Management' in the search bar and choose Colour Management in the list that comes up.
You should get a dialogue that shows your monitor and the currently set ICC color profile for that monitor. As a test change it to sRGB IEC61966-2.1 (or if you have a wide gamut monitor set it to Adobe RGB(1998)). To change it click 'Add' select the new profile then click Set as Default. You will now need to close and restart Photoshop. If that improves the issue then it shows you did have a defective monitor profile and you should make a new profile using a hardware calibration device. If it does not, then set the previous profile as the default again and close and restart Photoshop, to return to to the previous position.

Dave 

@DoYouLikeHam It's either your Photoshop prferences are corrupted or your monitor isn't perfectly calibrated, or if the default profile your system is using is generic or inaccurate, Photoshop will try to compensate for it. This compensation can sometimes make pure FFFFFF look slightly off-white (like FFFCDD) on your screen, even though the underlying pixel data is indeed FFFFFF. SnagIt is grabbing the color after this display translation. Also, Photoshop has a feature called "Proof Colors" (View > Proof Colors or Ctrl/Cmd + Y). If this is accidentally enabled, it simulates how your image would look on a different device (like a specific printer with a certain paper profile). This can dramatically shift colors, including white, to show what's "in gamut" for that device. If your Proof Setup is configured for a print profile, "white" might appear off-white to show what the paper's white point would be. For Photoshop, this meticulous color handling ensures that if you send your FFFFFF file to a professional printer, it prints as the purest white that printer and paper can achieve, regardless of how it looked on your specific screen.

For Illustrator and PowerPoint (and many other non-color-critical applications) often use much simpler, or even no, color management by default. They tend to just send the raw RGB values (like FFFFFF) directly to your graphics card, which then sends them to your monitor without much interpretation based on profiles. This means what you see is a "direct" display of FFFFFF based on your monitor's hardware, without Photoshop's profiling getting in the way. For general design and office work, this is usually fine.

So, how do you get your Photoshop up to FFFFFF? One, try resetting your Photoshop prefernces. Granted, it will default to the original settings, so, if you have any settings that you use all the time, make sure you make a copy of that! If that works, you are done... but if not, then Check Photoshop's Color Settings (Edit > Color Settings): Under Settings: For general web and screen work, "North America Web/Internet" or "sRGB IEC61966-2.1" is usually a good starting point. Make sure your Working Spaces (RGB) is set to sRGB IEC61966-2.1. This is the most common color space for web and general display. Under Color Management Policies, set "Missing Profiles" to Ask When Opening and "Profile Mismatches" to Ask When Opening. This will alert you if a file's embedded profile differs from your working space.

Next, verify Your Document's Color Profile (Image > Assign Profile / Convert to Profile), by making sure the Photoshop document you are working on (even a brand new one) has a profile assigned, preferably sRGB IEC61966-2.1. Go to Image > Mode > Assign Profile... and make sure "Working RGB: sRGB IEC61966-2.1" is selected, or if you need to convert a profile, use Image > Convert to Profile... and set the Destination Space to sRGB.

Also Disable "Proof Colors": Go to View > Proof Colors and ensure it is unchecked.