Ideas
Every time EA adds another pack, it ends up messing up something that was already in and established. Fortunately, I didn't have this issue until I was editing a lot I downloaded from the Gallery. I liked the shape of the house I downloaded but I knew I didn't have the packs so I downloaded an empty house. I went into build mode and there were no pink lines but as soon as I added a porch, there they were.
Luckily, I only had the pink lines on the porch but after googling, I found I could not remove the lines the recommended way because I don't have the pack that introduced this nonsense. I don't have a green or any other colored button between the move button and bulldozer button. Pressing o or O also does nothing. I read elsewhere that one has to move one's mods and run repair; no, I am not doing that, it's too much work, plus I run my game from the steam launcher so "repair" is not an option. I do have validate files which I suppose is the same but, still not doing that.
I found the only way to get rid of it (so far) is to exit out of build mode and then go back in and the pink lines are gone. Hopefully this works for anyone else experiencing pink lines. Good luck.