How to distinguish the earth from zero if they are the same color?

As an option - turn off the ground or zero on the input. You can apply voltage to zero, the main thing is to remove the jumper in the place of separation of zero (if you have TN-C-S). These issues are discussed in more detail in this article.
https://my.electricianexp.com/en/pravila-opredeleniya-fazy-nulya-i-zazemleniya-v-seti.html