I have not watched the show and do not know the type of light you are referring.
I've been involved with facility design and updating of juvenile, adult and animal confinement facilities.
If they are indicator type color can be if the call is vacant (un assigned or assigned), occupied, occupied asleep, and perhaps number of "tenants".
Newer cells may also indicate type of "tenant" violent or not.
There is also a push to give inmates comfortable lighting (pink light to not annoy and other color for nighttime so as to not disturb sleep pattern.
