We had this same problem and after wasting everyone's time troubleshooting we found it was this poorly implemented "Automatic camera brightness" adjust feature. DO NOT USE this feature. Do not use Webex, if you have the option. This "Automatic camera brightness" option is usually automatically selected upon startup. So right out of the box your video quality is going to look terrible. Disable this option!!!!!
This auto camera brightness added a nasty green hue to our video feed. Of course, we are not your normal Webex user. We are feeding the Webex meeting via web presenter from our video mixer. All the video and computer inputs are mixed on the video mixer and we feed our program output to a Blackmagic web presenter which presents a "webcam" to WebEx. We expend quite a bit of effort in maintaining video quality and, frankly, we do not normally use Webex (this is one of many reasons why). (We only use Webex occasionally depending on our audience requirements.)
When using Zoom (which we usually use) we do not have any color shifts or image degradation.
My point in posting this issue to help other content producers avoid this terrible color shift issue. A video streaming program is not supposed to make your video feed look worse. So if your WebEx stream is looking terrible and your output is just fine, then the issue is most likely what I referenced above as WebEx's poorly implemented and sloppy code in what they refer to as "adjust auto camera brightness." DISABLE DISABLE DISABLE