From my experience TEA and glycol based developers keep best when they were not overly heated during mixing. What I do is add the ingredients to the solvent and then with stirring slowly warm the mixture until everything dissolves. For some formulations you may not even have to heat the mixtures if you are patient. It may take a few hours or a day or so for everything to go into solution. Just add everything to a glass bottle, cap and gently shake periodically.
The darkly colored solutions may still develop film but may not be as active as fresh developer. The dark color comes from oxidized (and inactive) oxidation products of the developing agents.
Of the common developing agents pyrogallol has poor resistence to oxidation. Probably amidol is the only one with less resistence to oxiidation. Also phenidone slowly degrades in alkaline solutions. A better choice for 510 Pyro would be Dimezone S which was developed for better stability.
While these developers have good keeping properties they are nowhere near as stable as Rodinal or HC-110. Unfortunately there is a manufacturer of a Rodinal formula that is shipped in plastic bottles that do not promote stability to oxidation. This product goes bad very quickly unless it is transfered to a glass bottle. Agfa when it changed from glass to plastic used a composite bottle which blocked oxygen rather well. Still these bottles were not as good as glass ones.
The purpose of developers is to develop film not to be in a competition as to which one keeps best. The ingredients for 510 Pyro are relatively cheap and it is easy to make a new batch when there are doubts.