Hi Emmanuel,
Think of a light greenish glass bottle, spray paint it red. Take some sand paper and work the red tone off. Now you have a red bottle (opaque) with areas that are transparent and show these recycling green. The bottom part of the bottle is a massive glass cylinder, here the glass is not that transparent, but also not opaque at all. But maybe dark-bluish green.
There you have all three colors.
Surface colors and material colors mustn’t be the same, as in prints on the surface or labels, etc. The material editor separates attributes, and with that, I call it the shopping list, when you explore objects. Go one time through, keep that list in mind when you explore real world objects. And not everything in 3D has to be realistic, we do a lot of SciFi with 3D as well, and VFX and what not…
After all, the separation of the channels allows you to set up a wide variety of materials. Each channel has specific characteristics, e.g., Color is different to Diffusion and certainly different to transparency or even Reflectance. Illumination stands out as a unique feature, and Bump and Displacement have their own qualities, not to forget the Normal and other channels. Your questions points out that you explore this and the more you see the difference, the better you can handle the set up. Hence why I suggest to use the “Shopping List” and why I’m not a big fan of ready to use material (they leave you on a lower level of your skills, as anything else that doesn’t require basic knowledge).
In the attached scene, change the Absorption Distance to 2cm. This is a sketch and for demo purposes only, hence it is rough.
Let me know if that answers your questions.
