Some of the wire that I've had for 20 years plus has changed color in the insulation only, used to be a clear green, now its a dark cloudy brown, however there is no signs of corrosion in the copper wire itself. 25 foot run with the opposite ends twisted together still measure less than 1 ohm with a meter. Very common situation with some of the lesser grades of insulation used on some wires, the only real concern is when the wire starts getting brittle and the insulation cracks easily, then its time to replace it before a short happens.