Why is Wandering Son in watercolor? Not only does it represent childhood and innocence, but the colored pictures to the manga are often in water color.
Yuki has gotten a lot more feminine looking, right? Aside from growing her hair out, ditching her makeup and wig, and Art Evolution, she's a most likely on Hormone Replacement Therapy, so of course she's going to look more feminine over time. Just look at any bio-girl that's gone through puberty, or a lot of trans women over time.