A. The colour changes noted do not mean the food is unsafe to eat. Spoilage, however, may also cause colour changes. Therefore before you use any preserved food that has an unusual colour, examine it carefully.
What type of colour has it gone? Darkening of the tops of most preserved foods is relatively normal over a period of storage time. However, if the colour is both dark or black and maybe has a bluish hue to it or other colour hue like a rainbow, I would be suspicious of it personally.
What does it smell like? If it doesn't smell right the chances are it isn't.
If the first two checks are not right then don't taste it, dispose of it by emptying the contents of the jar into a pan along with the jar and lids, cover with hot water and bring the pan to the boil. Cover and boil it hard for 30 minutes at least. Cool and discard.
Wash the jar (s) in hot soapy water and sterilize immediately.