@millieee44 I'm glad to hear this worked. For the terrain paint not sticking, it's unfortunately yet another known issue:
https://answers.ea.com/t5/Technical-Issues-Mac/CURRENT-ISSUE-64-bit-Terrain-paints-not-applying/m-p/9667984#M12459
If you spray the terrain that's turned brown with a green paint (even though it doesn't appear to be sticking), then save, quit, and reload, does the ground look green again? You can use "save as" to rename the save if you don't want your saves to be permanently affected.
It's also possible that this is a side-effect of disabling Advanced Rendering, as that does change how terrain is rendered. I don't think it should all be brown though. If applying a new terrain paint doesn't help, try using the eraser, again quitting and reloading to see the effects. Going to Edit Town and back into live mode may or may not allow you to see the changes, so you can try that too if quitting and reloading gets too tiresome.