The leaves do not actually need to change colors.
Leaves have a "top" that faces the sun as well as a "bottom" that is in the shade. The plant cells on the top of a leaf are very dense with chloroform so they will have a dark green color while the cells on the underside have very little chloroform so the leaf will be a lighter green or grey-green color on that side.
Also consider that some plants are heliotropic (or phototropic) meaning that they twist to keep their leaves and/or flowers facing the sun. Sunflowers are the best known example of this.
Plants that open and close their leaves/flowers in response to darkness are called nyctinastic. Other plants will fold or close their leaves in response to other stimuli such as some types of mimosa that will close it's leaves when touched so that it is less attractive to grazing herbivores.
Combine the above properties and you have trees that will appear to change color based on which side of the leaves is visible to the viewer.
For example:
* If the native climate for the trees is hot and dry then the trees might only open the leaves to get sunlight during morning and evening when the sun is out but temperatures are low. 106
At night when there is no sunlight and during mid-afternoon when the temps reach their peaks the leaves will fold closed to prevent moisture loss. These trees would appear dark green at morning and evening but at night and midday they would appear to be a silvery grey-green color that tends to reflect light.
* At extreme latitudes (polar regions) where days can be short with the sun staying low on the horizon, trees might evolve to keep the topside of the leaves pointed at the sun to maximize the chloroform's exposure to sunlight. These trees would appear dark green when viewed from the sunny side but a much lighter green when viewed from the other.
* If the area has bad wind storms then the trees may have evolved to close the leaves whenever the wind is strong enough to make the branches sway, protecting the leaves from being damaged or stripped off by the wind. These trees would also close their leaves and appear to change color if a person or animal were climbing the tree and caused a branch to sway. There could also be an interesting visual wave of color across a forest in response to a low flying aircraft disturbing the branches.
* Tree with delicate leaves might close the leaves similar to the mimosa I mentioned above. 731