Why does the conch change colors?

The conch changes colors because it is removed from its natural environment and uncared for by the boys.

"The conch was covered in soot, forgotten by Ralph and Piggy, who's only thought was to keep the fire from going out."

I bought a conch shell from shop. Its polisbed and im a hindu .I used to add tulsi water in it and pray I found ut changes color from purple outside and brown inside