Cold blackening on steel turns yellow

I am having a yellow-green coloration when cold blackening steel injector bodies. please help

The discoloration could be from either contamination directly i.e. something is present in the bath that should not be or the bath having been contaminated and one of the chemical constituents having changed to a different compound. The black room temperature baths are based upon selenium chemistry and oxidizing agents with a host of other materials used to fine tune the process. First see if you make up a small quantity of new solution does the problem go away? If so, this is what you need to do and it is either depletion of some chemical component preferential to the others present or contamination. If this does not solve the problem look at how you are cleaning the part before you blacken. You should have a clean, water break free surface that then should be given a mild acidic dip before blackening to activate.

Of course you could simplify some of this troubleshooting by first asking your supplier for technical advise.