Maybe there's a logical answer to this or not - why is it when adjusting the Redsky / Bluesky settings on the Main tab or Tweaks of the Atmosphere Shader, they act in unison: when one goes in one direction, the other goes in the opposite. Way back in the Terragen Classic days we could adjust them individually, but since Terragen 2 for some reason it has not been possible. Unless there's a way I haven't found to turn it off, it'd sure be nice to have that option in a future TG release.
This has come up before: https://planetside.co.uk/forums/index.php/topic,26054.msg258805.html#msg258805 (https://planetside.co.uk/forums/index.php/topic,26054.msg258805.html#msg258805). Matt has some tips in his response there that might help.