Colors appear to our eyes what is reflected back from any object and give colors depending on that. When there is variation of thickness, material/object absord/ reflect different wavelengths and it results in different colors. 

Thanks

Dr. Alexey Stepanov and Dr. Savita Maurya

Can you please elaborate why the coatings of the same material but having different thicknesses absorb/ reflect different wavelengths. Is this because of different structure or different electronic transitions (band gap energy)?

It is the light interference effect. You can find correlation between the colour and the coating thickness on internet.

Ok

Thanks Dr. Pawel Kochmanski

Daer Dr. Watto:

                These are interesting questions at about Color Theory in materials.

                The classical phenomenon involving color development on a surface after some kind of recovering  is typically  based on the constructive/destructive interferences phenomena occurring between upper and lower surfaces of the film, as previously mentioned, that should exhibit proper thickness, that is flexible, see file attached. Here, it is interesting to define thin and thick, see link below:

https://www.researchgate.net/post/What_is_the_Difference_between_Thin_Film_and_Thick_film

also these features are coupled with deposition methods, consider see first attached file.

The theory is based on refraction index (substrate, thin and medium of stemming of the light), wavelength of light.

But, I think that understood your question in correlation to electronic transitions and its correlation wyh the parameter called optical-gap, since it is really possible to tailoring some kind of color using a very thin film at around the thickness equivalent to the nanoparticle diameter, possible to reach via magnetron sputtering. Phenomenon called of “color due electronic band structure” by  Furrer A. in 2013; consider read discussion of Figure 96.  

In above sense, is fact that neither all class of thin type metallic-alloy based on the nanograin might exhibit this phenomenon that is complex with some recent possibilities to be exploited.  Here, seems that defect at nanoparticle (nanograin of thin) are the key to understand colors at some thin films of specific material, see consider reach a good number of key-words at about this topic in the second attached file.

Best regards

Marcos Nobre

Besides all what has been said by all the above authors, these thin coatings are colored due to the interference between the light rays reflected from the outer and inner faces of the film, and their color changes with sample thickness.

Therefore, due to anisotropy of growth phenomenon, when you have deposited TiAlN coatings by magnetron sputtering of various thicknesses, different colors appear on the different material grains in the area of the interference film and are delimited by the contours thereof.

Further, there are actually two very nice discussions on the topic following the links below:

http://physics.stackexchange.com/questions/211396/how-does-thin-film-interference-work

http://physics.stackexchange.com/questions/143755/why-does-thin-film-interference-manifest-visibly-only-on-thin-films?rq=1

Thank you very much for such a nice description.

Prof.  Dr. Marcos Augusto Lima Nobre and Prof. Dr. Mohamed Akbi