In March I purchased a late 2015 5k iMac for myself, mostly to replace my 2013 MacBook Pro. Until around 3 weeks ago it worked perfectly, no complaints. I restarted the iMac one night, the screen appeared to go black but then a green screen appeared with black lines through the centre. The iMac restarted and thought nothing of it. iMac powered on, used it and then switched off. The next day I switched on my iMac, used it and then powered off. The green screen appeared again. So I decided to do a fresh OS install. A couple of days had passed and after a couple of restarts, I was then greeted with a blue screen with purple lines. I then decided to contact Apple Support, followed their advice with the PRAM reset, SMC and fresh install. This didn't solve my issue so I took it into my local Apple Store (frustrating as I don't drive!). In store they decided that the logic board had failed and needed replacing, so they did. After 6 days of waiting, I got the call to pick it up. The advised the logic board had been repaired, the iMac had been 'stress' tested and all was ok. This afternoon I've just restarted my iMac and hello blue and purple screen. Any idea's what could be causing this if it's not the logic board? I'm so tired of having to complain to Apple, my 2013 MacBook Pro had it's logic board replaced twice.