I need to calculate the reflectivity of a thin film, designed for one wavelength $\lambda_1$, at a different wavelength $\lambda_2$. The challenge is that $\lambda_2 \gg \lambda_1$, for example a factor of 100, so that $d \ll \lambda_2$, ($d$ is the film thickness). For example, would a $d=\lambda/1000$ film still reflect light? And if it would, then what is the refractive index of such thin film? Can I assume that it's the same as for a thicker film? Down to which film thickness would this approximation hold? Is there perhaps something like effective refractive index (as with optical fibres)?
As @JonCuster said, the formulas for thin film reflection will work fine (almost) regardless of the wavelength. So use the same Fresnel formulas.
The challenge will potentially be finding the correct refractive indices to use for the medium. Do not assume n is constant over such a large wavelength range; it is almost certainly not. Look in references and the literature to find n for your wavelength. Or better yet, compute it yourself from the results of your reflection/transmission measurements!