It may happen that e-liquids present a colouration ranging from a slight colour to a darker colour. This phenomenon is a function of the type of aroma utilised and the technology or technologies of manufacture (chemical synthesis, cold extraction, extraction by means of solvents, distillation, etc.). Take the natural aroma of blueberry, for example: it may well possess the blue-violet colour of blueberries, or it may be utterly transparent, depending on how the aroma was obtained (extraction by means of alcohol/water type of solvents or by biotechnological means with the use of enzymes). Some pure aromas, like the base Tobacco 1975, may be brown, which gives a very dark appearance to the liquid.
What is more, the colour of an e-liquid may possibly evolve over time, along with the chemical interactions with the nicotine and/or the oxidation of the product.
This chemical reaction in no way alters the taste-related and vapological properties of the e-liquid.