Fresh Glycin should be a light golden colour. Pale yellowy brown. (I've got some here of unknown age that's the colour of well-tanned caucasian skin, and it's still OK.)(posted 8345 days ago)
Any darkening is due to oxidation, and means that some of the reducing power of the developing agent has gone. You can compensate by using more in the formula, but how much more would be a matter of trial and error.
All developing agents are reducers, meaning they readily combine with oxygen, and halides, to 'reduce' metallic compounds to pure metals. This also means that they're susceptible to absorbing oxygen from the air, and rendering themselves useless over time. Some agents need to be in solution to work effectively, and these ones keep well in dry form. Phenidone and Hydroquinone have a very good shelf life. Metol and Glycin keep moderately well, while Amidol, Pyro and Paraminophenol are probably the worst for self-oxidation.I'd get the shop to give you some sort of warranty that the stuff is still good, or your money back, or you can try to haggle for a bargain price.