Light pink on bottom shell

[Patskyy]

Hi guys I have a 5 month old Greek spur thighed. I noticed today he has light pink on the middle line down his bottom shell as well as some pink on the top. Is this growth or is this something I should be worried about? He’s eating fine, moving fine etc.

 

[ZEROPILOT]

What type of substrate do you use?
The color is mostly in the crevices of the plastron

 

[Patskyy]

I use top soil