In a polycrystalline material, which includes the majority of tonnage of commercial metals and even many polymers, there will be a distribution of crystal orientations within even a small cross section. Some materials have nominally randomly oriented crystals (grains) within them. Others may have a preferred orientation, or "texture," so that most of the grains are oriented within a few degrees of a common axis. There are all kinds of variations in between.
Since the rate of film growth is different for individual crystals that are cut at different orientations to the axis of the atomic structure, differently oriented crystals will have different thicknesses of the film that forms on it. Since the thickness of the oxide is on the order of the wavelength of visible light, the oxide acts in a metaphorical way as a color filter. Each different thickness will appear to be a different color. In this way, we can see how much variation in crystal orientation there is in a solid piece of metal. Randomly oriented grains will tend to have larger variation in perceived color than fields of view with the grains all in similar orientations.
The particular colors that we see will also depend on the fidelity of the image-capture technique. We have already seen in the earlier posts how hard it can be to get "true color" in microphotos. In this case, it's not like the visible world where we know that a red-delicious apple is red and Granny Smith is greenish yellow. It's not like we know that the taxi cabs in New York City all used to be that deep yellow. In this case, the individual crystals are so small that they can only be seen at all with a microscope. So the scientist/ artist/engineer can have some freedom to make the photo attractive (or suggest something in the visible world) and select whatever colors they want for their specific purpose.
When I was in "metallurgy school," my freshman Materials Science professor claimed that fabric prints for clothing were sometimes taken from microstructure images. It was only 15 years later that I realized that the brown and blue and white design on my curtains (that I had been looking at day after day for five years) were different magnifications of unetched gray iron microstructures. It was graphite flake patterns that I was looking at. Years later, some pajamas I found had a pattern that looked like overetched annealed stainless steel, although in researching images for this post, I saw one that looked very similar and was identified as a biological tissue (beef muscle) sample. Learning to recognize patterns in one area of knowledge that you learned as part of another skill set is a useful thing to be able to do. It makes it easier to expand your knowledge into fields related to those you know the best.
Evaluation of microstructure photos is basically an exercise in pattern recognition. Figure 3, also from George VanderVoort's website is a color micrograph showing graphite nodules in ductile iron. Since it is unetched, the iron matrix is of a uniform color, which depends on the light settings of the microscope. The different colors of the "rays" of graphite within the spheroidal nodules are showing us different orientations of individual graphite films that together make up the nodule. So, here, it's the graphite itself, rather than a film on the surface, that is facilitating the multi-color display. Figure 4 is how graphite nodules in ductile iron usually appear with regular gray-scale metallography. This specimen has been etched, so the microstructure of the iron-rich matrix is seen to be partially pearlite. Nodule A clearly shows similar features to the ones in Figure 3. Maybe this nodule just "happened" to be oriented favorably to show these features without special lighting techniques.
In any case, when I saw the image shown in Figure 3, without having read the caption, my first thought was that it looked like the polypropylene "spherulites" that I grew in thin films as part of a college research project to study the rate of crystallization of thermoplastics. Learning to see patterns in nature, even when they are depicted at vastly different scales, is a life-enriching experience.