When I mix my Pyrocat HC it initially turns a very light ble/green colour then clears. However if the Metabisulphite has decomposed completely Pyrocat does turn a more noticeable blue/green which doesn't clear and this is an indication that the Pyrocatechin has started to oxidise and it'll stain the negatives often unevenly. It's a bit subjective but a slight colour on mixing isn't an issue but if it's quite noticeable then I'd discard and make fresh Part A.
My first batch was stored in small Polythene bottles and went off very quickly a couple of months as the polythene was low density and allowed oxygen migration through the walls. In decent "High Density" bottles I have Pyrocat HD that's lasted over 2 years (no glycol). The fresher the Metabisulphite the longer these developers last as stock or in a dish/tank during processing.
When I use IT-8 a Pyrocatechin re-developing toner the developer has a remarkably short tray life and once it turns in colour stains the prints patchily.