A little long explanation. The basic summary is in bold.
I am working on an animation. The clouds are animated too. I used easy clouds.
I had many parameters animated.
But then i saw some problematic funky high frequency color highlights and strange colours in general in some images.
I tried to animate some other parts to overcome this problem. But one cloud was stubborn.
I deleted one by one the animated parts to find the problem(searched actually first in the whole scene nodes).
I copied the problematic easy cloud to a default scene to show the problem (hopefully with the other values at kinda default).
Looks like animating the "Cloud depth" is the problem here (Renders are from frame 4000. The problem shows up in other frames too.).
But if you change the first animation keyframe (2500) to any value other then zero it looks ok.
Even with negative values in frame 2500,frame 4000 rendered without a problem.
The examples below are all rendered from frame 4000. Terragen renders the same frame very different sometimes.
Terragen v.4.1.21. Window 10 64 bit.