iPhone 6 losing contrast and fading colors

hi

i've an issue my iphone suddenly losing contrast and fading color.. at first I thought it was software issues but after I tried to restart iPhone, tick untick invert colour, even downgrade iOS to 8.3 (I'm using 8.4 before). The problem still exist and it become worse.. the screen dramatically losing its colour and my phone looks like when I play Pokemon green in gbc! Anyone has the same problem?

what should I do??

thanks before