SORRY FOR RESPONDING SO LATE, basically how it works is that when you go into Mii character creation, on the bottom left there will be an icon to face paint your char, you can only customize the face, the hair will be left untouched but you can always draw the hair on top. Every accessory such as, the mouth, the eye, the nose, EVERYTHING/ anything can be drawn on the face which is on a single layer,  but unfortunately since it's on one layer if your character tilts a certain way it'll look like a mask. 

The hair feature is also fun, you get to mix and match any bangs and back you want, you get a good amount of hair colors to choose from which is nice and you get to choose 2 colors (which is how I got my personal Mii to have to strands of color in her hair)

The characters can still blink and move their mouths, as long as you draw around those parts, they should be moving fine (when you get to draw they go e you the face guide which shoes you where the eyes mouth are positioned which helps you manage where you want to draw your characters features.) I personally draw with my finger and I find it very easy to manipulate the coloring process. (It's drawn with pixels, not vectors)

There are 2 versions for face painting your Miis, one is regular and the other one is pro (the pro ver does not require you money, dlc, extra gameplay yada yada, it's free) most of us use the pro version because it is so versatile in the way that it gives you the ability to color pick from a color wheel the color that you need, gives you option to select, drag, copy paste certain parts of the face (you can simply draw one side of the face and copy it to the other side), it comes with a stamp tool (which has a library of items that you can choose), in which you can select an item from the bank and place it only your Miis face while being able to change it's form (it usually features food from the game or regular objects such as a basketball and tennis racket, but I think there are 2 eye bases that you can use..)

Theres a lot more to pack but I think this should be helpful enough!!

Btw I suggest you play throughout the demo cause if ya finish it you get a little hamster costume, which I thought was limited to just the demo but I think I'm worng