Physics-soap bubble

As a soap bubble (n = 1.33) evaporates and thins, the reflected colors gradually disappear. (Assume that visible light has a wavelength range of 400 – 700 nm.) (a) What is its thickness just as the last vestige of color vanishes? Not 300, 300.75, 150, nor 150.38. b. What is the last color that is shown?