Rodinal will turn brown very quickly but this will not affect it's working properties. However I don't like using a chemical that appears to have gone off so what I do is decant the 500cc bottle into 5 x 100cc glass bottles (or the equivalent) and use one at a time. I don't know where you will get them where you, are but my local pharmacy will sell them very cheaply once you explain what they are for.

I also use them to preserve RA4 Developer and Paper developer which I make up or buy in large quantities. This stops the chemical from deteriorating. Plastic will absorb oxygen over time and this is what 'kills' the developers. Glass is a far better alternative.