The use of coloured dark glass is necessary when bottling premium extra virgin olive oil because it helps protect its contents. Clear glass is more easily penetrated by light and temperature changes, whereas green, black, brown and other dark colours are all much more resistant. Oils bottled in clear glass may have preservatives added to increase their life duration.
2 Best before date
Extra virgin olive oil is a natural product, and like all things in nature it deteriorates over time. A quality bottle of extra virgin olive oil will proudly announce its origin and optimal lifespan and has a "pressing" date or sell-by date (usually one year after it is pressed) on the label, so you will know the age of the product. Unlike wine, olive oil does not age well. If you store it properly, in a cool, dark place away from light and heat, a good bottle of olive oil will not go rancid during its time in your kitchen. You can even wrap the bottle in aluminium foil to shield it from further sunlight.
It is always a good sign when a label says the olives have been cold pressed. This means no heat was applied during the crushing process to extract the oil from the olives, which avoids changes in the olive’s chemistry and potential defects. Adding heat to the olives allows producers to extract more oil from the olives but simultaneously destroys the delicate flavours and aromas which result in a good extra virgin olive oil. It should be noted that "cold pressed" means that the temperature does not exceed the 27 °C; not actually "cold."
Well-made extra virgin olive oil will never be dirt cheap. If an “extra virgin” olive oil is much more affordable than the other ones on the shelf, that’s a red flag. To produce the best quality of olive oil takes time and money and high-quality olive oil producers have certain costs to cover before they can even start to make a profit. So, in this case you get what you are paying for.
Good extra virgin olive oil should be grown, pressed and bottled in a single country. "Product of Italy" does not necessarily indicate that the olives are grown or pressed in Italy, only that it was packaged there. It is well known that Greek producers sell tones of olive oil to the Italian producers which they later label it as "Italian".
Choose for the oil quality, not the colour. Despite our natural inclination to associate a rich, deep colour with better flavour and quality, the actual colour of olive oil is not an accurate judge of its quality. In fact, extra virgin olive oils can range in colour from deep grassy green to bright, yellowy gold. This variance is due only to the level of chlorophyll in the olives when they are pressed.