by Scorpion0x17
There's one card that spawns the Abomination at Orange too.And one that spawns him at Red.
In other words, there's one card per level that spawns the Abomination.
This, combined with the way the number-of-kills-per-level-increase goes up, means that you have to be really unlucky to get him at Blue, slightly less unlucky to get him at Yellow, less unlucky still to get him at Orange, and you're gonna get him sooner or later at Red.