1. Here's why
The Autumn Colors Effect will override the tree and plant colors in all other modes if you click on a Photo slot or Clip with that effect, or if Lumion renders a Photo slot or a Clip with the Autumn Colors Effect.
This is a technical constraint as Lumion does not store a copy of the tree and plant colors in each mode.
2.1: Adjust the tree and plant colors as desired in Build Mode.
2.2: Assign an Autumn Colors Effect to an unused Photo slot or Clip, but do not adjust the sliders in the effect:
2.3: Then click on that Photo slot or Clip before returning to Build Mode.
When you assign the Autumn Colors Effect to a Photo slot or Clip you are saving a snapshot of the current tree and plant colors which can be loaded again any time you click on the Photo slot or Clip.