Why does my phone screen go rainbow?

So, this has been happening to my iPhone 6 for a while now but it's just started to get worse. It began with the screen brightness flickering, then small rainbow fuzzy spots would appear, and now the whole screen is basically not able to be used. It sometimes goes back to normal after I turn it off and on, but after a while it will happen again. Will Apple warranty cover this? I have only had the phone for a year and 3 months. The bottom of the screen was cracked a while ago but never affected anything, I just didn't get it replaced because I don't live near an Apple store. Thanks in advance!