I looked at my mission log a while ago, and i noticed that one of my successes was gold instead of green/black

What does this mean?

I'm not completely sure, but I do know there are layers of success based on the pet's stats...

Looking at my mission log (which has all 100% successes on the current archive, since my mission pets are well-established and don't fail unless I do something like accidentally sending one on the wrong mission), it looks like green is the best: you get 2 mission satchels and whatever amount of gold.

I know you can succeed but not perfectly, where you only get 1 mission satchel and less gold... so perhaps another success is technically passing the mission, but doing so poorly that you get no rewards...?

Red = Fail = no reward
Yellow = Poor = half rewards, no mission satchels
Black = Satisfactory = base reward, one mission satchel
Green = Exceptional = double reward, two mission satchels

Thanks