What causes changes in the colours of soap or oil films for a given beam of light?

Light is incident on a glass plate $(\mu = 1.5)$ at an angle of $60^o$. A dark fringe is observed corresponding to a wavelength of $6000 \, \mathring{A}$. If the thickness of the glass plate is $1.2 \times 10^{-3} \, mm$, find the order of the interference fringe.

$A$ mixture of yellow light of wavelength $580 \ nm$ and blue light of wavelength $450 \ nm$ is incident normally on an air film of thickness $2.9 \times 10^{-4} \ mm$. The colour of reflected light is

$A$ thin film of thickness $t$ and refractive index $1.33$ is coated on glass with a refractive index of $1.50$. What is the minimum thickness $t$ required for strong reflection when light of wavelength $600 \, nm$ is incident normally?.......$nm$

This question has a paragraph followed by two statements,Statement $- 1$ and Statement $- 2$. Of the given four alternatives after the statements,choose the one that describes the statements.
$A$ thin air film is formed by putting the convex surface of a plano-convex lens over a plane glass plate. With monochromatic light,this film gives an interference pattern due to light reflected from the top (convex) surface and the bottom (glass plate) surface of the film.
Statement $- 1$: When light reflects from the air-glass plate interface,the reflected wave suffers a phase change of $\pi$.
Statement $- 2$: The centre of the interference pattern is dark.

The approximate thickness of an oil film required to observe the interference of light (due to which it appears coloured) is .......