The color combination for the first 3 fragments inserted does not matter. The dragon spawns are based on the color of the 4th fragment inserted. Once a dragon has been defeated, the option to use a fragment of that same color as the 4th insert will go away and you will no longer be able to spawn it again in that same instance.

Ventani: x-x-x-Gold

Nanzata: x-x-x-Red

Tukaarak: x-x-x-Black

Hraashhna: x-x-x-Blue

The trick is just which rune you put in last. If you put the blue one in last you get this mob. Just have to make sure the last rune you put in the second time is not the same as the first time.

The catch is, you want to fully control what dragon you spawn. Each one drops only one pair of legs and they will always drop the same pair for archtype. Example is, this one will always drop scout pants.

You get 4 types of shards, 2 of each, right? 2 blue 2 black 2 yellow and 2 red.
So either way you put it, you use 4 shards to spawn 1 dragon. And we got 2 dragons, just dont know yet what is the logic in the method of spawning them. Ty for the help !

We have spawned him 3 times now using the order I posted. Gold, Black, Red, and Blue. If you use two of the same color, you mess up your chance to spawn two Eidolons. You only get 2 of each color stone on one run through of the raid zone.