Basically, Yin is Black while Yang is white. Yin is negative while Yang is positive. Two opposite thing yet they kinda coexist to form the base of existence. It's like how there is good and bad and why everything that has an advantage also has a disadvantage. So Yin relates to spiritual energy while yang relates to physical energy. Coming together, they form what are know as chakra though they can exist independently and can also coexist.
So Yin-Yang are basically the primordial forces of the universe. But they weren't the first. Before there was Yin-Yang, there was nothingness. You've seen these before on this site in two major ways: LoK's sage title (the Sage of Wuji), and Reborn's Hayabusa clan, based on Wuji. Think of Wuji as two stages before Yin and Yang, and a state of true nothingness. Wuji is defined as the boundless and infinite. It is literally, in Chinese, translated to "without ridgepole," meaning there is no direction, no poles (think North and South poles), etc. In Naruto, this state was represented as Kaguya Otsutsuki herself. The end of Naruto was laden with very rich symbolism, despite how the story kinda went to wild shit. It's something I really do have to give props to Kishi for. Basically Hamura and Hagoromo, the Six Paths Yin and Yang powers, respectively, represent Yin and Yang. This is also represented by Naruto and Sasuke, and Hashirama and Madara. Then when these powers combine, they become Taiji (the next state I will talk about). This is Madara after he declares he is immortal, basically, and how he has obtained "both powers." Then you have Kaguya. She represents the ultimate state. Had she succeeded it would have, in a way, been a return to nothingness.
Now from Wuji comes Taiji. Taiji is translated as "Great Pole," and it is the state of the Supreme Ultimate. Taiji is the state of everything. It is the primordial universe, with infinite potential and boundless boundaries. For some, thinking about what Taiji is can be weird. Taiji is a state of undifferentiated everything. It is a single great pole that encompasses the entire universe. This is where Yin and Yang come in, which is the next state. Yin and Yang are poles that allow Taiji to be differentiated. It gives more definition to a universe of everything. Yin and Yang is further broken down into the 8 Trigrams, or the Ba Gua. These 8 Trigrams combine in a variety of different ways to give us the Creation of 10,000 Things; this essentially is, in philosophy, what creates everything in the universe. In the Naruto RP, this is represented as the Creation of All Things Technique, creating living beings from nothing. The Ba Gua does this by utilizing Yin and Yang in a variety of different ratios.
We'll talk about ratios soon, as it is important in Yin-Yang techniques. But essentially, and it was pointed out above, that one cannot exist without the other. That means that in all Yin and Yang techniques, the opposite, to some degree, is still present. This is true even in the ultimate Yin and Yang techniques in the RP: Yin State and Yang State. In these techniques you will reduce your Yang ratio (in the case of Yin State) to the absolute minimum. But this does not mean that the Yang ceases to exist. As we go through the techniques we're going to identify, in as many as we can, where the Yang is in each one. This is a Yin training thread - but we'll be looking at it in the context of Yin-Yang. Because no matter how much you try to remove the Yang, it is always present.
So you've mentioned some basic characteristics that define Yin-Yang. You mention positive for Yang, negative for Yin. They coexist, yet they're also opposites and compete with each other. They are always seeking balance. This is where ratios come in, but like I said we'll cover that later on. So what do you think are characteristics that define Yin physically in the RP?