Force Elder (guardians) to spawn on red maps?

How do I do that? He always spawns on yellow maps for me. What is the trick to it?

Do I really have to get elder influence on all red maps and then cut off the yellow maps without reaching 20 influenced maps? But this is not possible afaik (not enough red connected maps).

The more red maps you have, the better the odds, yes.
Add to that the fact that naturally, higher tiers are slightly more likely to be chosen.
So yeah, you might have been unlucky there.
Cheers =]