Color is an accurate guide to the state of Dektol.
It begins to lose activity when it turns medium yellow - you will need to increase development time to get to maximum black the paper can deliver.
You can still use it when it has turned orange, but developing times may stretch to 10 minutes or more. A tablespoon of carbonate can bring expired Dektol back to life again for a few more prints.
When it starts to turn brown and smelly it is far gone and should be flushed.
Expiring developers can be used with papers because paper is developed to completion - you just keep the paper in the developer until it is finished.
However, when making soft prints the print isn't developed to completion. To prevent streaking and splodging soft-working developers work very slowly, allowing you to pull the print before it is completely developed. Many photographers used to keep a jug of old Dektol on hand that they would mix 50:50 with fresh developer to make a slow working 'soft developer'.
It is, of course, possible to poison Dektol so it doesn't work while remaining clear - pouring the stop tray back into the developer bottle will do it.