Unlike a lot of other foods, olives don't seem to have undergone many radical changes during cultivation from their wild ancestors, though the exact history of domestication is still somewhat of a mystery. Olive tree can also be exceptionally long lived if properly cared for, with some that are centuries old still being harvested today.

Even in ancient times there were different 'grades' of oil, varying in quality and taste. Just like today, unscrupulous merchants might try to stretch or adulterate expensive oil with cheaper varieties for more profit, to the point that the Roman Empire had laws about how oil could be sold and inspectors to enforce it.

Several Roman authors wrote about the necessary properties of 'good oil' and there was definitely an emphasis on quality for those who could afford it. The Roman method for extracting oil was considered so superior that it hardly changed for a millennium. Many small batch or artisanal producers use essentially the same methods.

All this to say there is likely not a large difference between similar levels of quality from then to today.

Extra Virginity: The Sublime and Scandalous World of Olive Oil is a book with a lot of interesting information about the history and uses of oil, though it is rather more of an entertainment style of writing than historical research.

Olive oil in Antiquity was categorised by grade, and quality olive oil was a highly sought-after food product for its superior taste. Some regions and cultivars also had varied reputations for the quality of their olive oil.

Book 12 of Columella's Res Rustica ('On Agriculture') is a prominent primary source authority on olive varieties in Antiquity along with the process for olive oil production.

For one, Col. lists numerous varieties of olives, which each have different styles of preparation, preservation and utility that he goes on to describe. This included the Pausean, orchite, shuttle, and royal olives. (Col. Rust. 12.49.2)

Any yield of olive-oil produced in the first half of the olive-harvesting season is very small, and produces an olive oil that Col. refers to as 'summer oil,' which is very bitter. Only olives that have fallen from the trees due to storms and need to be collected before wildlife pick them away should be turned into summer oil.

To answer the question of colour, Col. distinguishes between 'green oil' and 'ripe oil.' Both requires the further ripening of the olives, which begins to bear fruit in the latter half of the olive-harvest season starting around December. Similar to the modern olive process, the less-than-gold-ripened green olive oil is by far the most valued, and can double a cultivar's harvesting profits, but while the yield is greater than that of 'summer oil,' it's lesser than the yield of oil produced from fully-ripened olives. (Col. Rust. 12.52)

For taste, beyond those parameters, it's unlikely that ancient olive oil alone would have a significant divergence in taste compared to modern varieties. One caveat is that the flavour profile of olive oil is indeed shaped by factors such as the timing of harvest/production, weather/heat control, storage, and the production process itself (eg. how the olives are pressed, using a mill or a press, etc.). Compared to the better-stored modern olives which are pressed with the uniformity of industrial processing equipment, the lack of these by cultivars in Antiquity combined with an overall cooler climate during that time period could have potentially accounted to some small difference in taste across the board.

Sources:

Columella, Res Rustica 12.

Kron, G. 2015. 'Agriculture' in Wilkins, Nadeau, eds., A Companion to Food in the Ancient World. Malden. 168.