Siddha Bhagwan (liberated souls) have no body, no skin, and therefore no color. In Jain philosophy, Siddhas are formless, bodiless, and free from all material qualities like color, taste, smell, and touch. Both Digambar and Shwetambar traditions agree on this.

If you are asking about symbolic representation: in the Navpad (Navkar) tradition, the Siddha-pad is represented by the color red (symbolic only, not a skin color). | For more on the bodiless state of Siddhas at Siddhashila: