I don't have a lot of experience with this, but the limited I do have was not good.
I was given a half filled bottle of DD-X concetrate, age unknown. It was lighter than light straw in colour.
A test roll developed to the recommended time at the specified dilution came out very weak. I did not want to try to calibrate the small volume left in the container, so I safely disposed of it.
So for this developer, my experience is that colour is not a good indicator of its vitality.