After replacing my thermostat in my 2011 E350, I needed to add back in some coolant. The original coolant is green, and I only had some yellow Zerex G-05, which is also for Mercedes.

Now, I just read that mixing green and orange coolant will cause it to gel and mess up your engine. I'm not sure if the yellow coolant will do the same.

Did I do a big goof?

yellow is for models up to june or july of 2011 i believe , so its a NO

so before that they used a blue one , if yours is green i would assume it was manufactured after those dates

Aside from the question whether or not to mix different coolants (no opinion here), once mixed, the result will be a different color, neither green or yellow. Not bad if you can remember which coolants they were... I assume quite bad if you can't.