The color has lost its brightness and is a brownish shade of the original color. Can I still eat the jam or jelly and can I prevent this from happening in the future?
Low-sweetener jam or jelly that is a bright color at first will begin to fade over time and with exposure to light. This is a process of color loss and does not mean the jam or jelly is going bad. The browning starts at the top of the jar and slowly works its way down. If your jars were properly sealed and the seal is still intact, the jam or jelly, although not as pretty as it once was, is safe to eat.
To slow down the process of color loss, store your sealed jars in total darkness. You can also add some lemon juice even when a recipe does not require it (1 tablespoon per cup of mashed fruit or juice at the most). Freezing your fruit and then making the jam or jelly closer to the time when you are going to eat it or give it away is also recommended.
Using citric acid powder instead of lemon juice can be more effective at preventing color loss/color change. One-quarter teaspoon powdered Citric Acid is equivalent to 1 Tablespoon lemon juice. 1 teaspoon powdered Citric Acid is equivalent to ¼ cup lemon juice (4 Tablespoons). Citric Acid lowers pH (makes the mixture more acid as does lemon juice) and imparts tartness to the fruit mixture (as does lemon juice), but it doesn’t add a particular flavor.