I recently cracked my HTC One M8 but it was still working. I have since put in a replacement screen which didn't work so they sent me another one but still the same problem. The screen is multi-coloured pixels and nothing else
Either the connectors are bad, the cable is pinched, the motherboard is bad or you did something wrong. It's not something that can be diagnosed without having the phone in hand. All that's working is the backlight, and there's some noise going to the LCD screen. (It's possible, though not likely, that all the company you bought the screen from has is bad screens. They can buy them cheap and make some money from people who take the phone into a repair shop after the first attempt fails.)
mine does the same thing after a replacement and it only happened like a couple months after a successful installation. its something wrong with the screen itself, because i unplugged and replugged the connectors and it still happens.