OK, so I'll share what I've had so far. I'm a lvl 16. I have all but the rainbow & I've tried all the different suggestions so far, needless to say it's still random I can mix the same pair 3 times and get 3 different dragons or 3 of the same, no rhyme or reason, guess it's just luck.
Here are the ones I've had success reproducing most of the time with these pairs, but no guarantees, good luck!
MUD - ice, earth
MOSS - plant, mountain
TREE - plant, earth
FLOWER - plant, fire
SONIC - air, lightning
SNOW - air, cold
MOUNTAIN - earth, cold
STORM - cold, lightning
Hope these work as well for you all as they have for me!