How to unlock orange heroes?

Orange heroes are not unlocked. You can recruit them in the Heroes Hall building with recruitment tickets. Unfortunately it is random and the chance rate of an orange hero is low. You can recruit then with any ticket but the Orange super recruitment tickets have the best chance (still low though).

You can also get special tickets from some events that guarantee an orange hero, or pay for some extra packages that also give an orange hero or tickets.

There is also the option to earn hero tokens from daily challenges and once you have enough you can buy hero fragments in the heroes hall (token shop) and combine them for heroes. All of this takes time though as orange heroes are designed to be hard to get.