Answer
The light resulting in fully constructive interference shifts toward longer wavelengths.
Work Step by Step
Note that light in the visible range has a wavelength of approximately $400~nm$ to $700~nm$
Note that there is no phase shift due to reflection because the index of refraction of each layer is greater than the index of refraction of the layer below it.
Since we are looking for a maximum (fully constructive interference), the path length difference must be $~~(m)~\frac{\lambda}{1.70}~~$, where $m$ is an integer.
We can find find an expression for the required wavelength $\lambda$:
$2L = \frac{(m)~\lambda}{1.70},$ where $m$ is an integer
$\lambda = \frac{2~L~(1.70)}{(m)}$
Since the thickness of the film $L$ increases, the light resulting in fully constructive interference shifts toward longer wavelengths.
