Some FO's will have a color change but still be ok to use...the only rule I use is if it doesn't smell the same way it used to when I first purchased it, I may not be so quick to use it..I have a few that are beginning to smell a bit different than they did so I may end up just doing a small test to see..if it doesn't smell right then its sadly going down the drain
Unless the FO is water soluble (and the ones I use are not), I would not put them down the drain. The oil-soluble ones will float on water and many are flammable. In an enclosed sewer system with the methane that is generated by the decomposing wastes, the potential for a serious fire or explosion is high. If you've ever heard about sewer fires when someone puts motor oil or gasoline into a drain or back in the day when people put flammable dry cleaning solvents down the sewer ... same idea.
I would put the FO in an open disposable container and let the FO evaporate into the open air in a safe place away from kids and pets. Wrap any remaining residues well and put them in the trash. Or take them to your county or city hazardous waste disposal.
Many of the MSDS sheets for the FO's I've read also say they're harmful to fish and aquatic ecosystems (the dead fish with a tree in the background). I'm keeping my FO's until I have enough to take them to the county hazardous waste disposal. But, I work in a chemical plant where disposing something down the drain that isn't supposed to go down is a very very very bad thing. Its made me question what I dispose of in my own house.
I've never actually had an FO go bad on me, even the ones I bought ten years ago, when I was first going to get into soaping and had to set aside because of life reasons, I only found them again last year