You should calibrate your monitor
It has been calibrated using a Spyder. But this is YETI different appearances on one monitor, anyway, so I don't really see how that would cause the problem.
...two different appearances, rather. Really, autocorrect? YETI?
Try disabling the graphics processor by unchecking Preferences > Use Graphics Processor.
Thanks. You know, I actually did end up doing that yesterday, and it seemed to help a bit, but I'm still having issues. I found a thread that indicated my video card is just hopelessly incompatible with Lightroom 6 (Radeon 6800 HD).
I have a hard time seeing how using the graphics processor, or not, can affect the color of an image. I also don't see how a "hopelessly incompatible" graphics processor can affect the color of an image. The graphics processor could affect the speed at which the image is drawn.
I do want to go back to the statement
It has been calibrated using a Spyder.
When was it calibrated? If this calibration happened a long time ago (or even a week ago), you might want to try calibrating the monitor again, because calibration can affect the color of the image.
Absolutely! But the same image as it appears twice on the same screen?
I think it might be possible. But in the time you have spent arguing against calibrating the monitor since I first suggested it, you could have calibrated the monitor and within 10 minutes, you would see with your own eyes if it helped.
Well, I have two monitors and both are calibrated every 30 days. The last time being about two weeks ago. Of course, I never actually argued against calibrating monitors. Thanks for trying, however.
I agree with dj_paige's suggestion to double-check your calibration. An even quicker check than re-calibrating is to set temporarily your display profile to sRGB: How do I change my monitor profile to check whether it’s corrupted?
Now Lightroom is crashing regularly when I move between Library and Develop, and sometimes the image previews aren't showing up at all. I think it's a video card issue. But I will recalibrate for safety, and try the sRGB thing, as it certainly can't hurt. Thanks!
how using the graphics processor, or not, can affect the color of an image.
LR 6.0 / CC 2015.0 had one such a bug: Lightroom 6: Rendering problem with saturated colors using Graphics Processor. This particular bug was claimed to be fixed in LR 6.1 / CC 2015.1, but it does illustrate the possibility that the graphics processor can affect color. So disabling the graphics processor is a quick diagnostic.
Are you on 6.1?
I don't have a deep understanding of GPUs or how LR uses the GPU. But reading Eric Chan's comments, the GPU is doing at least some color processing. It may be that LR is using the GPU to apply the display profile's color transformations.
Are you on 6.1?
Triple-check this by doing Help > System Info. The recent problems with LR installation and update has fooled many people into thinking they were running an updated/upgraded LR when they weren't.
6.3, it says:
Lightroom version: CC 2015.1 [ 1025654 ]
License: Creative Cloud
Operating system: Windows 8.1 Business Edition
Version: 6.3 
Application architecture: x64
System architecture: x64
Logical processor count: 8
Processor speed: 4.0 GHz
Built-in memory: 32693.6 MB
Real memory available to Lightroom: 32693.6 MB
Real memory used by Lightroom: 1697.0 MB (5.1%)
Virtual memory used by Lightroom: 1680.5 MB
Memory cache size: 2090.9 MB
Maximum thread count used by Camera Raw: 4
Camera Raw SIMD optimization: SSE2,AVX,AVX2
System DPI setting: 96 DPI
Desktop composition enabled: Yes
Displays: 1) 1920x1080, 2) 1920x1200
Input types: Multitouch: No, Integrated touch: No, Integrated pen: Yes, External touch: No, External pen: Yes, Keyboard: No
Graphics Processor Info:
AMD Radeon HD 6800 Series
Check OpenGL support: Passed
Vendor: ATI Technologies Inc.
Version: 3.3.13397 Core Profile Context 15.200.1046.0
Renderer: AMD Radeon HD 6800 Series
My previews are both the correct (matching) color at the moment, actually, though I haven't had a chance to do any of your troubleshooting yet (I disabled using the graphics processor a while back, however, and maybe it needed to refresh a cache or something). Lightroom is just totally unstable when I try to edit now. Freezes when I click around in Develop.
You're actually on LR CC 2015.1 (the most recent Creative Cloud version). The "Version: 6.3" refers to the operating-system version. In Adobe's current terminology, "Lightroom 6" refers to the perpetual-licensed, non-subscription version, which is missing the cloud features (e.g. LR Mobile), and which receives all the same bug fixes as LR CC but not any new features (Dehaze being the only such new feature so far).
Ah! Thank you. So I should stick with this version?
Yes, it appears that nearly everyone is better off being on the most recent version (CC 2015.1).
Well, after calibrating monitors afresh, deleting caches and rebuilding previews, uninstalling and reinstalling Lightroom (twice!), I was still having the crashing/freezing plus disappearing and miscolored previews issues. So, I bought and installed a new video card. Things seem to be running smoothly now, at last. Thanks for all the input, both of you!
Glad you're up and running. It's unfortunate you had to go through all this pain.