The labels of transparent, semi transparent, and opaque on the back of an oil paint tube used to be something that I ignored. In part because I just didn't understand how to utilize this information to improve my painting process; I paint in an alla prima technique, and I ignorantly believed that these labels were only important when layering or glazing. But after realizing how untrue this was, my painting process quickly improved.
When considering transparent vs opaque, its first important to understand how oil paint is structured. Oil paint is made of two parts, the pigment and vehicle. Pigment being what gives it colors, and vehicle is what holds it together. Pigments are tiny particles of a material or mineral, and before they are mixed with the vehicle/ oil, are much like dust or sand. Generally, opaque colors will have pigments that have larger particle's. As particle size increases, Opacity increases. As particle size decreases, Opacity decreases and transparency increases... When mixing an opaque red and blue you often get a greyer muddier color, vs when mixing a transparent red and blue, you often get a dark violet. This is true for most pigments and colors, and of course there are exceptions to this rule.
This is important to know because it has helped me learn what I/ you can expect when mixing colors. With learning what to expect I have also gained more control over my pallet.
Another general yet very important and simple rule regarding opacity and transparency is that generally transparent colors recede and opaque colors come forward.
My process for learning these rules typically begins with an experience/ experiment, I may notice something when painting,, experiment with it for a time, then make my way to looking up what I learned. For me, having this hands on first approach helps to solidify these concepts. but how ever you go about it, its important to understand and learn how to apply these sorts of concepts to your process. There is alot more to opacity and transparency that i haven't included in here, but if you are interested in hearing more on my thoughts on these, feel free to reach out ! Thank you for taking the time to read this!