Prius transmissions do not
Prius transmissions do not have clutches. There are NO friction materials in them what so ever.
This means the fluid should NEVER darken unless there is something wrong.
EX: If it is brown, it has been overheated.
If it is blackened in the least little bit, walk away! The surfaces of the gear teeth have been worn off or there is a major lubrication problem.
PRIUS TRANSAXLES CAN'T BE REBUILT!!
There are NO repair parts available and your ONLY options are to put a used one in from a low mileage wreck or buy a new one (if it's available).