Flavorings are a mix of volatile and semi-volatile substances which give the desired aroma and taste to finish a flavor.
Taste and aroma go hand in hand. For example; if you have a cold, food and drinks taste different since your nose can't pick up the aroma.
The raw materials used to create our flavorings have an inherent color. These raw materials react with one another and colors change. Sometimes this reaction takes a long time and you don't notice, while other times it could take a few months to only a few weeks for this reaction and you will notice a change in color.
Lastly, the amount of nicotine will also react with the flavorings and you may notice a difference in color when you compare a light strength to an extra strength cartridge.
The changes in color in no way affect the taste or the aroma, these changes are a normal occurrence.