Pumpkin seed oil has an intriguing and unusual optical property; it is dichomatic. A thin layer is coloured some shade of green but a thicker layer is a dark-reddish purple. When I heard about this from Steve Mold's YouTube piece The rare property of pumpkin seed oil - dichromatism, I bought some and took this photograph.
The camera has distorted the colour of the green area slightly; it is less yellow and more green that the photograph suggests, at least to my vision. EDIT here is another photo, that shows the two colours better.
From what I can gather the explanation is that a thin layer of pumpkin seed oil transmits significant amounts of blue, green, yellow and red light and the blue, green and yellow together dominate and are perceived as green. However once the layer gets more than 0.7 mm thick the blue, green and yellow are sharply attenuated and the transmitted light is nearly all red. I owe this information to the Wikepedia article on the topic at https://en.wikipedia.org/wiki/Dichromatism
Next week I'm going to try an Austrian salad dressing, pumpkin seed oil and cider vinegar.