Hi guys,
I believe I've fixed (or managed to avoid the cause of) my issue. I've been testing for 10-15 days and no sign of problems yet.
My scenario is pretty specific though, but who knows, it might help someone.
As you can see from my previous post (screenshots above), I had a pretty bad "Techicolor Rainbow" case, that occurred when waking up from sleep from time to time.
My Mac Mini is set up to launch XBMC (Kodi) after boot and is pretty much always on. I also set up sleep in XBMC (after 20 mins of idle state) and I made XBMC turn off my screen after 5 mins on top of that (I have plasma screen). After it goes to sleep, after a couple of hours it also goes to "deep sleep", as planned.
After the 10.10.2 OS X update I no longer had problems waking up, but the colour issues remained (as you can see above).
What I realized in the end was, that the cause of colour issues was the screen being turned OFF before entering sleep state. If Mac goes to sleep with screen ON the problem disappears. By turning off this setting in XBMC (now my screen only goes black after 5 mins - doesn't actually cut the HDMI signal) my Mac goes to sleep normally and wakes up without any issues. I can also wake it from sleep and deep sleep via remote without problems.
I hope this help someone 🙂