Paint your master key?

[Stone Teran]

I have a few master keys from landlordlocks that look identical and I can't tell them apart on my keyring.  How do I differentiate these keys?

The keys say do not duplicate, so the local locksmith doesn't want to cut new ones.  They are a bigger key than normal, so I don't know if those new walk-up kiosks can duplicate it. I could paint it with rustoleum or nail polish but I don't know if it'll rub off.  I'm sure there's a simple solution to this that I'm missing.