Turbo snail shell

[Baby Damsel 219]

I have posted this before and wondered why my 2 turbo's shells on the front are yellow but the rest of the shell is its original color. Eats move had for like 9 months. Any tips of explanation will it go back to normal?

 

[Dan_P]

I have posted this before and wondered why my 2 turbo's shells on the front are yellow but the rest of the shell is its original color. Eats move had for like 9 months. Any tips of explanation will it go back to normal?

Picture under white light would help.