I do purple zip ties because it's an odd color (although after reading FT apparently not as uncommon as I thought!). I keep the spares and a nail clipper in my carry on to replace for the trip home. Having purple zip ties came in handy on my last trip because I could tell right away that my bag was opened as it had a replacement black zip tie on it. I opened it immediately (with the nail clippers from my carry on) and found the love letter from tsa. Although it occurred to me- far fetched as the premise is- a thief or smuggler could always open it afterwards and steal/smuggle but at least having the different color zip ties gives me a heads up.