If you don't want to place them all over again just use the Change Properties button, select them all (hide all the other sheets first), and change the colour to a more appropriate green.
Did you place the trees after changing the color? The preview on the mouse cursor doesn't account for the varicolor color of the trees, so the preview will still show grey-ish trees until you place them. Also, existing trees are not changed if you just change the color on the status bar, but you can use Change Properties to change the color of existing trees.
Comments
Choose a color like red, and if the trees start placing as red, you will know thats the reason.
If you don't want to place them all over again just use the Change Properties button, select them all (hide all the other sheets first), and change the colour to a more appropriate green.