@dortis
I don't think the colour or order is relevant, you might just be getting unlucky.
I haven't played in like six months or more and when I found out there was an update I came to check it out. Missed loads of unique abilities and Rux deals but no point crying over spilt milk.
I managed to get the red and yellow keys in two consecutive weeks. I may get all four straight away or maybe I need to spend another half a year getting the other two pieces...all I can do is wish you good luck for the next one.
I don't like the idea of the "75% chance" to get a piece either, I think the chests respawn more frequently so that you can open it multiple times for the chance of a key. There's no downside to that except the ones who get unlucky have to play more, but not getting a key must be extremely frustrating when others have unlocked the characters and are crying out for buffs and nerfs.
If I were you, you should make a forum suggestion stating that they should respawn, or can be opened multiple times each week. I don't see why someone can't grind the love out of the game and get all four keys in one week now that it's been out a while. Once you've made the suggestion link it on their Twitter so they see it.
Edit: Actually I'll do it for you, if you or someone links it on their Twitter. 🤔