Tagging @eywaschild891, in case you were still interested in this!
(note: this tutorial is about the digital process of adding the stripes, not actually designing the patterns)
I'm using MediBang Paint Pro, but this technique should work in any program that supports layers.
So you've drawn your Na'vi and now you want to add some stripes! First we'll have to start with a base color. Underneath your lineart layer, add two new layers: one as the main base color layer (for coloring hair, eyes, clothes, etc), and on top of that, a separate layer for the skin. Putting the skin on a separate layer is important because that way you'll be able to do the stripes as a clipping mask.
Your layer box should look something like this (you can ignore the gray background and extra folder, they're not important for this tutorial):
Now we can start on the base stripe layer. Create a new layer and set it as a clipping mask on your skin layer. This makes it so that you can't "color outside the lines", so to speak, while drawing the stripes. Then take a hard-edged brush and start drawing the patterns you want. I like to use MediBang's "mapping pen", which has a subtly textured edge, but the regular default Pen tool will work perfectly fine as well.
If you'd like, you can stop here and call this done: solid stripes are perfectly acceptable in a cartoonified stylization. However, if you look closely at Na'vi stripes on canon characters, you'll note that they're not completely solid: they have a subtle fading effect, where they're darker along the edges and a bit lighter in the middle, and in some places they fade out into the body. Sometimes it almost looks watercolor-ish.
(I'm sure there are better examples out there than these but I'm not up to digging for them rn 😅)
So, let's continue! To fade our stripes, create another clipping mask layer above the main stripe layer. Use a brush with pressure-based opacity such as MediBang's "pencil" to loosely fill the insides of the stripes with the base skin color.
Once you have filled in all the stripes, use the gaussian blur filter to smooth out the fade. The filter doesn't need to be super strong—in fact, if you make it too strong you will loose the detail and the stripes will look solid again. With MediBang's version of the tool, I had it set to 7 (out of a max of 64) for this image, though of course the way it works may be different in other programs; mess around with different settings until you get something you like.
(sadly I do not have a screenshot of the blurred version because apparently the way my cintiq takes screenshots is dumb; it only captured the "gaussian blur" popup window instead of the whole screen. oh well. :P)
Now that we have the fade within the stripes, let's also make them fade into the rest of the body. The areas you'll want to do this with are the torso, inner arms, and inner/back of legs.
Create another clipping mask. Using the same pressure-opacity brush and base skin color as before, loosely block in where you want the stripes to fade into the body.
Once you've finished, gaussian blur the layer just like before. This one can be a little stronger than the inside-stripe one. For this image I had it set to 14.
Sometimes, the areas you block out will overlap with areas that shouldn't be faded—for example, here the fading from the back of the character's thigh is overlapping with her tail. To fix this, just erase it to remove the fade from where it shouldn't be (this is why we do this step on separate layers). Make sure you do this erasing after you've applied the gaussian blur.
Depending on the pose of your character, there may be places where one faded area overlaps with another. For example, in this drawing, the blocking for the arm fading overlaps with the blocking for the torso fading. Here it is with the torso fading turned off so you can see the overlap:
This makes it difficult to do both the arm and the torso on the same layer, because I won't be able to remove where the arm fading overlaps the torso fading without also removing the actual torso fading.
Luckily, there's a simple fix: just do the arm fading on a separate layer. Do this as many times with as many layers as you need to for your particular pose. Sometimes you'll only need one or two, like here; other times you'll need several.
BUT, fading the stripes is the last step! Regardless of how many layers you end up needing, once you're done, you're done! Now you have a Na'vi with nice natural-looking striping 😸
At this point you can go ahead and merge all the clipping masks into the main skin layer and even merge the skin layer with the other-base-colors layer if you're ready!
Hopefully that wasn't too hard to follow; feel free to ask if anything is confusing or unclear. Happy drawing! 😸