1. similar system is not the same as "this exact set of colors". My point is that it requires us to memorize completely random color scheme.
2. what ARPGs use this exact color scheme?
Any system would have some randomness to it. Replace "Orange" with "Gold" as it is meant to be, and the order used here is very common.
World of Warcraft:
Gray-White-
Green-
Blue-
Purple-
Orange-
Gold-
Light Blue
Star Trek Online:
Gray-
Green-
Blue-
Purple-
Dark Purple-
Gold
Star Wars The Old Republic:
Gray-White-
Green-
Blue-
Orange-
Purple-
Yellowish Orange-
Yellow
Final Fantasy XIV: White-Pink-
Green-
Blue-
Purple
Mighty Party: White-
Green-
Blue-
Purple-
Gold
Lost Ark: White-
Green-
Blue-
Purple-
Gold-
Bronze (sorry, lack of good color in the ones suggested)
Runescape: White-
Green-
Blue-
Purple-
Orange
Hearthstone: (none)-White-
Blue-
Purple-
Gold
So, yeah, none a 100% fit, but any player who dabbles in this kind of game will likely be accustomed to the white-green-blue progression even if unsure at first why that is.
It is a convention, just like certain keyboard layouts - you could argue that QWERTY is stupid, and many people do, but it is generally visible and people will learn it sooner or later.
That doesn't mean that additional symbols or similar couldn't be helpful to add, but as for the color scheme itself, it ain't broken, it is pretty much standard in video games. (Physical CCG like Magic the Gathering have different systems, agreed)
Also, the rarity doesn't really matter, a card does what it does, you cannot really upgrade it or similar. You cannot trade them in for anything and care about some sell value. Yes, it may be a bit more prudent to get the higher rarity items first to not let the RNG get a chance to run total amok later, but otherwise what is important is what the card does for your playstyle, and that is written out clearly. So I consider this a non-issue in any case.