you are not supposed to farm fates for them, go to OC and only if you are missing one or two get the rest from fates.
I would honestly say the droprate inside oC is ... 'fine' but the problem is that they are zone specific drops, so i got 12 orange and only one purple atma. at the end
if you still need bicolored gemstones, thats also a good oportunity for farm these and do the relic.