I’m going to keep this brief and pretty to the point.
Based on the colour schemes for Yamato on the volume cover of WSJ 46 and Toki in the anime, I see similarities that got me thinking they could've been inherited even though pale hair is recessive. It’s only partially, so it could be strange combination of Ancient Kingdom DNA and oni/dragon DNA having that effect to allow the variegation through the brown. Kaido’s hair is pale in dragon form as well. I get it, you’re saying when/how could this have happened? Well remember in chapter 968 when they discuss how Kaido’s followers infiltrated Kuri castle? They make a point of Toki using her body as a shield. What if it’s not just the arrow that she took? She could’ve been forced to manipulate the fetus into the future inside herself to gestate and birth it quickly then send it off/have it taken from her.
So what I think happened is that Kaido showed up, had his way with her, under threat of harming her children; forced her to age the baby to gestation/give birth and took it leaving before to secure the child and trusting the underlings to handle the rest. He did this because, through his connections with Rox, he was aware of her status and connection to the Ancient Kingdom and coveted her bloodline.
The underlings tried to dispose of the other heirs to Wano and the scabbards returned in time to cause turmoil, allowing her to progress her healing, get in the way of the arrow and have them ne defeated/run off. Being the strong, stoic, secretive female that she is, she never mentioned the earlier occurrence before the scabbards arrived and the children were too young to know much or say anything. I think that Toki included the diary of Oden’s with Yamato’s swaddling in the hopes that it would somehow reach him and help to guide him on the correct path in life despite his father. I think she knew it wouldn’t be destroyed because the wet nurse would still be a citizen of Wano and recognize the connections thus do their best to keep the baby safe even low key.