Same problem here. Colors are only reliable in CPU mode, but that is horribly slow. GPU mode is completely useless while designing, because the colors are all washed out.

Changing color mode from standard (emulate adobe illustrator 6.0) to Europe General Purpose 3 fixed it for me it seems

@Juan Gómez
Thanks! Changing the color profile worked perfectly.
This started happening for me a couple/few weeks ago. I received a color profile warning around that time. I think it updated my profile to the Emulate Adobe Profile when I selected the accept option.

A guy called Nash emailed me about the same issue with a tip that might help you out if you have the same problem:

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I may have figured it out. My color settings were set to something funky. Once I changed the color settings to North American General Purpose 2

Edit > Color Settings > Settings

Was: Emulate Adobe Illustrator 6.0
Changed to: North America General Purpose 2

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I did it and mine was also to "Emulate..." so I changed to custom/standard color profiles "sRGB... & US Web Coated..."

Before this email I changed my iMac screen settings to the standard color profile "sRGB IEC61966-2.1" and that solved a little the issue but the problem was still visible; I also turned the brightness down since I have it mostly at 100% all the time, and noticed today that the difference was gone or imperceptible. Don't know which one helped, I turned the brightness back to 100% to see if this was the issue.

Having the same problem and I'm really mad at it; I didn't noticed at first since most work I did was in PS but recently found out about this and thought it was a problem with color profiles but it clearly isn't since you can see the variation on the color picker and the artboard, document is in RGB, also the line with a gradient showing on the screenshot too, shows "glitchy" issues that shouldn't be there and they're solved in CPU Use mode.

My device is:
iMac (Retina 5K, 27-inch, 2019)
6 Core 3.1 GHz Intel Core i5
16 GB 2667 MHz DDR4
Radeon Pro 575X 4 GB

Anyone else experiencing this issue could you please email me at jmgomezwilson@gmail.com in case there's no answer in here since it is an old thread?. Thanks.

I'm having the same issue. Turning off GPU acceleration improved colour accuracy immediately but that's a big sacrifice to make. This has to be a software issue since the previous version of Illustrator CC before the most recent update was working fine.

We have all the same problem here with our imac 5k we dont know how to fix that.

This problem persists and I can't rely on choosing colors in GPU preview mode. I can't work in CPU preview mode because the application slows down on a 2018 5k Imac to the point where it's unusable. I hope Adobe releases this a serious problem for professional users and attends to fixing this - it's not up to us to update our video cards!

i'm experiencing the exact same problem!