iPhone 6 plus flash and then gray?

Hi All,

I'm working for cell phone repair store. Recently, I have experienced 2 iPhones but have no idea what are wrong with them. One is iPhone 6s and one is iPhone 6 plus. If I luckily get the phones on normaly, then suddenly go blank grey(almost black) and no pictures or anything show... If I lift up screen, I saw the backlight is on but on screen show nothing, but grey(it's hard to see grey and at first I thought it was black). If I wait for a white, press home button or power button then the phone show normal then flash several times then go grey again. I have tried to put new screen but there is no help, things still the same. On iPhone 6s more wierd, sometimes I'm able to power it on to home screen(I said sometimes because most the time I powered it on and it go direct to blanks grey screen) but if I moved it around or little touch on screen, it will go blank grey again. Do you guy have any idea.

Thank you very much!