I see the same issue on my TheC64 Maxi but it's not just the foliage - the brown of tree trunks changes, the purple of the bricks changes.
Viewing post in Nixy The Glade Sprite (C64) comments
I noticed your observations after I posted my comments. The colour hue changes to a deeper shade when scrolling left or right and also vertically, up or down, and returns to an insipid hue when the Nixy character stops. I haven't noticed this effect on the YouTube reviews so conclude those videos were made with original machine or perhaps using the Vice emulator. The effects, as already mentioned, don't appear on my Android Frodo emulator. I'm guessing it must be the C64 Maxi emulation. At least I now know it's not just my C64 Maxi! Perhaps Psytronics might proffer an explanation. Thank you for your reply. BW, Michael