an ape's an ape, a varlet's a varlet, though they be clad in silk or scarlet

Definition 
The true nature of a person or thing may be hidden by outside appearance but cannot be changed: "The naked ape is in danger of … forgetting that beneath the surface gloss he is still very much a primate. ('An ape's an ape, a varlet's a varlet, though they be clad in silk or scarlet')" (Desmond Morris, The Naked Ape, 1967). The proverb is of ancient origin in the form "An ape is an ape, even if it has gold insignia," which occurs in the works of the Greek writer Lucian (c. 125–c. 200). The word varlet may mean "servant" or "rogue," and silk and scarlet are traditionally worn by holders of high office, such as in the legal profession, at whom the proverb is often directed.