Using the bragg's law the path difference for first reflected light, 

The thickness of film is $d=2.9\times {10}^{-4} \mathrm{mm}$

The angle between the incident ray and film is $90°$

Therefore, 

$$\begin{gathered} n\lambda =2d\sin \theta \\ \lambda =2d\sin \theta =2\times 2.9\times {10}^{-4} \mathrm{mm}\times \sin 90° \\ \lambda =5.8\times {10}^{-7} \mathrm{m}=580 \mathrm{nm} \end{gathered}$$

The wavelength obtained is corresponding to Yellow light.

Thus, the colour of reflected light is, Yellow.