So I've had a late 2015 5K iMac with wide-gamut P3 display for a couple weeks now. Pretty sweet machine, but color management can be an issue. Unless you're using color-managed apps, you're going to be seeing a lot of oversaturated colors. Safari (and Quicktime) look great. But other web browsers have issues. Here's my breakdown: Safari: Safari handles color correctly. ICC-tagged images are correct, and untagged images are presumed to be sRGB and get correctly mapped to the screen's gamut. Video content looks good. Chrome: Chrome displays tagged images correctly, but untagged images are oversaturated (which means most images and graphics will be oversaturated). Video content is also oversaturated, like Youtube, Netflix, and Hulu. Firefox: Firefox is a mess and oversaturates both tagged and untagged images and video. (Deeper Firefox discussion at bottom of post.) Bottom line is if you want a wide-gamut iMac, I hope you like using Safari. Other notes - VLC oversaturates videos. Thankfully you can change the saturation within the application, but I haven't been able to completely replicate how the videos look in Quicktime and it is a bit annoying. - Webpages can look small on this screen, and unfortunately Safari does not remember zoom levels from previous sessions. You got to re-zoom every new session. I do believe there are third-party extensions that can help, but frankly remembering zoom should be native. Chrome and Firefox remember zoom, but the colors are wrong... - Netflix does not stream 4K to computers, including the iMac. Total bummer that I didn't realize that until after I bought the machine. Firefox discussion Back to colors in Firefox. Firefox is oversaturating everything, tagged or untagged. I have gfx.color_management.mode set to 1, which should enable color management for tagged and untagged content. But it's not color-managing anything. I suspect that Firefox isn't able to read the iMac's display profile. I found a bug report for Chrome (https://bugs.chromium.org/p/chromium/issues/detail?id=562951) which sounds like a similar issue from not long ago. Apparently the iMac ICC display profile contains an unexpected value which caused Chrome to assume it was not a valid profile and thus ignore the profile completely. I'm not an expert, but that's how I read the bug report. I suspect something similar is happening to Firefox. There is a bug filed with Firefox already (https://bugzilla.mozilla.org/show_bug.cgi?id=1250461), but I'm not sure how much attention it's getting. If anyone has any other information on this issue, I'd love to hear about it because I'd love to be able to use FF again.