I have registered just to add to this thread. I suffered the same problem. I have an AMD RX580 and a fine card it is, but after a new install of Windows (and new AMD drivers) I turned it on this morning and my second monitor was all purple. To be fair it was more lacking green than being set to purple but you get the idea. Read the [TL;DR] at the bottom if you just want to try out the "fix".
I have a dual monitor setup, one has VGA and DVI inputs, the other has VGA, DVI and HDMI inputs. The RX580 has 3 (I think 3) HDMI outputs, 1 DVI and several DP ports.
Moving the cable from the second monitor to the primary one and it also displayed without green, when it was fine via HDMI, so this led me to believe it is either the cable or the card (either software or hardware) that were at fault.
I read another thread (and there are surprisingly few recent ones) that they fixed this with the nVidia control panel - albeit this was a comment from 2012 or something like that. So I went hunting in my Radeon drivers. I am currently using v19.7.1 (Win10 x64 Adrenalin) drivers. In the Radeon Settings dialogue there is an option to set the Display values - in this version of the software it's a button to the bottom right (3rd from the right in).
Clicking Display showed my two displays with the primary having more options. One of those was "Pixel Format" which was set to "YCbCr 4:4:4" however clicking it offered me 3 other options (one limited RGB, one full RGB and one with a different pixel ration). I chose the full RGB option, more in hope here, and it brought my green back!
Interestingly I can swap back and forth between them and none return to the purple. So in the end I have reverted it back to it's initial setting and it's fine. I put this down to a bug in the software. Now I have not rebooted since then but ultimately I don't need to worry that it's anything hardware (the monitor, the cable, the card) and that a simple software setting resolves it.
[TL;DR]
Open up Radeon Settings, select Display, on the primary monitor change the Pixel Format - doesn't matter what so long as it's changed. Your monitor will now return the green, and stop being purple/blue. You can return the Pixel Format back to it's original state (mine was "YCbCr 4:4:4") your monitor should not revert back to purple.
Hope this helps someone. It's not a problem I had encountered before and not one that seems to be discussed in forums (or certainly not one discussed recently).
