Currently I haven't found a way to go back to and change the player crest you make when you first start. I might be missing something, or it might simply not have been implemented yet, but I'm hoping you'll be able to change it later on, since for one reason or another you might want to alter it (like say if you start playing vikings a lot more than samurai you might want to change it to something more thematic, or you might notice a mistake you made in spacing that makes it look weird and want to change it for that reason)
Edit: Also I know there might be legitimate reasons for this, but a real purple would be nice in it. I like purple a lot as a color, but the closest things in the emblem maker are a vaguely purplish blue and very faded purple/pink colors. A few real purples to choose from would be nice.
You can. Press the button to go on social (triangle on PS4 and probably Y on XB) hit R1 to go to the next tab and there's a section for the emblems where you can create 4 and choose from them and edit your first by pressing square or (I'm assuming) X on XB
Also yeah the lack of a real purple is kind of annoying and the colours in general are a bit restrictive I think. I'd prefer just a proper colour chart like in other games.
The shape of the shield I think is based on which army you choose at the start and I'd guess is square for samurai, round for viking and kite for knights.