When looking straight down on a film of oil on water, you see the color red, How thick could the film be to cause this effect? How thick could it be at another place where you see violet?
I do not know the velocity of light (index of refraction, n) of this oil.
However figure the thickness of oil for constructive interference between the light reflected off the oil and off the water underneath for a red light wavelength and a violet wavelength in the oil.