It usually comes down to 5 things:
1) the actual color of the oleoresin inside the wood. Have you ever smelled blue oud oils? If you have, you'll notice that they ALL share a certain note in common which is absent in non-blue oud oils. That note is in fact similar to a facet of blue essential oils like Blue Tansy and Australian Blue Cypress.
And so, the first factor is the natural color of the oleoresin in the wood itself. Some are naturally yellowish, others greenish, and others reddish. Some are naturally lighter, others naturally darker.
2) the grind setting.
Typically, a finer powder will yield a darker oil due to more direct exposure of the oleoresin to heat, whereas with a rougher grind the oleoresin will be 'buffered' by the wood fibres, and thus the oil will typically be lighter.
3) directly related to the point above: the cooking temperature. In most cases, a higher temperature will give a lighter oil. Same reason as above^
4) pot size. Usually a smaller pot will be more efficient at conducting heat, allowing a lower cooking temperature = lighter color. Killer Koh Kong was cooked in a 3kg pot, and Vietnam Special K is being cooked in a 15kg pot. So... obviously this isn't always true (refer back to point #1).
5) copper vs steel. The former will typically give a lighter oil and the latter a darker oil. Not always the case (once again, Koh Kong was copper vs Ha Tinh is steel).
..........and a bunch of variables can be tinkered with, to counter each of the above.
So, in summary, yes
typically color is dictated by the above-mentioned techniques+apparatus variables. However, the way I do it, its not techniques+apparatus that determine the color but rather the oleoresin's natural color itself that does.
m.arif can tell you the Cambodian was cooked at a SUPER
low temperature, in a
TINY COPPER pot, so one would think the oil would be very light and yet its almost black.
By the way, the color of the Vietnamese oil's base note fractions isn't very accurate/clear in the photo above because its silhouetted against the brighter background. Here's another photo that better captures the color:
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