We can see the colours in a soap film due to three main factors-i.e
1) Size:The colour of the film depends on the thickness of the film. A bubble becomes thinner and thinner as it dries out(due to evapouration )before finally popping.As the surface film of the bubble becomes increasingly thinner, a change in overall colour can be seen.
2) Wavelength:As the bubble film gets thinner yellow wavelengths get cancelled out. As it gets more thinner green lights gets cancel out.Similarly due to shorter wavelengths blue colour also disappears.The resulting colours are the combination of the colours that do not undergo destructive interference and their degrees of constructive interference.
Eventually, film becomes to thin to create interference of visible wavelengths, as all wavelengths are cancelled out. At this point the bubble appears colorless. Against a black background the bubble surface could appear black.
3) Angle:We can see the colors in a soap film as the position of observer changes, the angle also changes, hence we can see different colors.