It's probably not the gamma, most likely some other change you made that you thought was unrelated. Simply changing the render resolution in your original scene will get rid of the yellow issue, so you could easily have changed something else that affected it.
It seems to be a little more complex than I thought, but what I can say is that copying the cloud node *alone* from your project into another one will reproduce it. So it does seem to be mostly related to the cloud settings. The issue does not reproduce itself in all circumstances, i.e. different render resolutions, details, or whatever, but I haven't seen it at all without that cloud node and its relatively high settings. So I'm pretty sure that's still the culprit one way or another.
- Oshyan