Hey Halfar! It's great to have you here! I'm also from Germany! If you don't mind i would like to contact you with a PM
RUB means = Red blue Black if i remember correctly, G means green in that case. I also wondered why this abbreviation is used, but i guess it's just a regular term in TCG.
It's based off of now outdated Magic: The Gathering Multicolor Terms.
U is used for Blue because "Blue we represent with the letter U. Oh, real quickly. Why do we do that? I’ve talked about this in my column, but for those that haven’t heard me say it, when first Richard(The Creator of Magic: The Gathering) made the cards, the problem was that black and blue both start with B. So why not go to L then? Well, land...card colors are used to signify the border, the kind of card it’s on. And so land has its own border. So L is for land. Well, why not go to A for black? Well, A is artifact. And so U was the first we could get to in either of them, so we ended up going with U." - Mark Rosewater
These terms are now outdated because new offical terms have since been released by Wizards.
For those who don't know these terms here they are:
Allied Colors: Azorius (White Blue), Dimir (Blue Black), Rakdos (Black Red), Gruul (Red Green), Selesnya (Green White)
Enemy Colors: Orzhov (White Black), Izzet (Blue Red), Golgari (Green Black), Boros (Red White), Simic (Blue Green)
3 Allied Colors: Bant (White Green Blue), Esper (Blue White Black), Grixis (Black Red Blue), Jund (Red Green Black), Naya (Green White Red)
2 Allies+Mutual Enemy Colors: Abzan (White Green Black), Jeskai (Blue White Red), Sultai (Black Blue Green), Mardu (Red Black White), Temur (Green Red Blue)
4 Colored does not have any terms to describe them yet that I know of.
5 Colored is called WUBRG (pronounced Whoo-Burg, again based off the mtg color wheel)