so far, rwby has mentioned two eye colors in particular; first one being silver eyes, which were first pointed out by ozpin (the only hint we ever got that something was up with silver eyes) in relation to ruby, and the same happened later with oscar. silver eyes only work in presence of and against grimm, and ruby inherited them from summer.
the other, is red eyes. red eyes were first brought up by yang in relation to grimm—"those burning red eyes"—and couple episodes later raven shows up, wearing a grimm mask with only her red eyes visible. red eyes were also mentioned by the waitress in higanbana while describing raven: "from the woman upstairs. red eyes. said you wouldn't mind bottom shelf." yang's eyes are purple—a mix of tai's blue and raven's red—but they do turn red when she uses her semblance or when she's angry; a negative emotion which grimm are drawn to.
so we have two eye colors, one that is very much anti-grimm, and the other that is associated with grimm.
then, we have the brother gods. both of them have been shown to change their form from human to dragon, but how they do it differs.
the god of light changes form in a flash of silver light, the same color that his eyes are in his dragon form. the flash destroys grimm, making the god of light progenitor of silver eyes. as such, his transformation is explicitly tied to the power of silver eyes.
the god of darkness, however, starts his shapeshifting on screen, sprouting wings and growing a tail and his horns larger before the rest happens in a puff of smoke. it's not just a one simple flash of light, despite the god of darkness gifting humanity magic—which can be inherited—in the first place; there's actual shapeshifting going on.
you know. shapeshifting. like the branwens do.
two sisters who could (and have, in ruby's case) inherited magic from their mothers. two brothers, who happen to showcase these specific magical feats. and would you look at in which god's domain ruby and yang are seen in:
in conclusion: birb!yang is real. yes this is about that in the most long-winded way possible. i'm not sorry.
now, i'm not saying that red eyes in particular are a sign of shapeshifting magic—i'm not sure if that would even really work since raven and qrow already have differing eye colors—but rather that these similarities and parallels are used as a way to hint that yang inherited magic as well, and how the god of darkness is, perhaps, a source of this magic.
as for how shapeshifting magic found its way to humanity 2.0 in such small quantities? who's to say. the brother gods know how to mess things up, i trust them to find a way.











