The Book-Ends involved with Shay and Vella in the beginning and endWARNING, THERE WILL BE SPOILERS
The beginning has Shay and Vella with their eyes closed, sleeping and oblivious to the world quite similar to how their character stands in the beginning. They're blank, dormant, and waiting. Their color schemes- blue for Shay and red/orange/pink for Vella -are surrounding them in an immense cocoon of familiarity. They're still in their element, still safe, still waiting.
Following their adventures along with all the mindblowing that ensues, we now see at they've traded places. Literally. Vella is in the remains of Shay's ship, and Shay is stranded in Shellmound. Their color schemes have followed them, Vella with her red and Shay with his blue, but we now see slight tints of the opposite in each. Shay's side now has a Orange/Blue Contrast with the sand and "Mog Chothra's" skin is predominantly blue with small bits of red in the shading. Vella has a bluish nebulae mingling with the red (which by the way has a remarkably... Dangerous tint to it now especially considering this is a wrecked ship). Also note the more frequent use of gray and black hues compared to the opening. Their eyes are now open. They have been exposed to each other's worlds and truths and their character has taken a large step forward with their adventures.
Another tie in with this is their arcs. While they're both going against what their parents want as part of their Coming-of-Age Story, they have differences in such. Shay is trying to be a "grown up" and Vella is trying to change her society. "I want to be treated as an adult." and "I want to defeat Mog Chothra and keep my family safe." are good ways to sum this up.
But now they're facing the choices they've made. Vella is facing Shay's broken reality (he wanted to be the hero who saved everybody and thought he was in space and perhaps even thought he was the Last of His Kind but both of those are either wrong or questioned) and Shay is facing what will likely be a cultural upheaval because of the "slaying" of Mog Chothra. At worst he could be attacked now that the pressure of the gigantic beast is dead, or at best he could end up like Alex as another "Dead Eye God". Either way they're about to deal with each other's problems and experience the consequences of their choices while they walk the roads to finding their own way and the answers they seek.
Who knows what Act 2 will bring, but I'm positive that it will involve these developments somehow.