The gray ones are polybutylene and I recommend that they be changed any time I come across them in a building.
Number 1 reason for a service call when it relates to a toilet supply leak.
They either bust at the bends or when they freeze, right above the nuts.
Can't expect much @ .38 cents a piece.
On one of my forums there was a spam post involving corrugated stainless tubing like CSST configuration.....3/8" and very easy to use.
Kinda like the older type flex copper ones we used to see years ago.
Can't find them in my area but I was curious to know if they were expensive or not.
I stick with Fluidmaster SS supplies; reliable and no reliability problems.
I can upsell the replacement of PEX supplies in the entire house by taking one off and showing people how easily they snap off right above the nut where the ferrule was tightening down on the pipe or at the faucet supply nut.
Compress/distort/strain plastic and it's a given it will harden right there where you did it.