Faceups Tips for painting bunny ears?

There are a couple ways you can tackle this, I will list them from "easiest" to "more advanced" keeping the materials you have in mind.

The basic route:
- seal ears
- use pastels to blush and lay down the base color, like a light brown if you want it to match the skintone better, and use other colors for shading as needed (like dark brown/black at the tips and a more rosy pink for the inside)
- seal again, repeat the above step as necessary, seal one last time before we do the hairs
You will have a better looking, more even and deeper color when you do several thin layers of sealant, blush, sealant, blush VS trying to do it all in one layer. At some point the resin surface cannot accept more pastel powder, and darker colors also tend to look more blotchy vs lighter ones layered on top of each other.

Once you have blushed the ears to a color of your liking, you can start painting on hairs. Look at real rabbit hairs to see the direction they grow in/wrap around the ears.
You can either draw them on gently with the watercolor pencils, or you can use a very fine brush. As a beginner doing it with a brush might be more difficult. Also even though the paint is already diluted you should dilute it further. Once again layering is key as well, since that way you can do hairs that are lighter/darker and it looks less crusty and thick. Make sure it is acrylic paint as well, and not lacquer or enamels (since those are popular within miniature circles as well).
You can also just do a couple hairs at the tip vs covering the full ear completely. Doing full hair coverage that actually looks like hair and not just blocky lines all over the piece is a bit of an intermediate to pro level endeavor.

The more advanced route:
You could maybe dye the ears before you do any painting. Dyeing means the base cover is a) more durable and b) laid down waaay more quickly than you can do it with pastels. You could also dip dye a gradient.
Basically first do a full dip to get them to a light brown, and then slowly dip in less and less of the ear to get the top part to look darker than the bottom one.
Depending on how well finished the ears were though, you might have some streaks/sanded areas become visible after the dyeing. You can even that out with the above mentioned pastels.
There are some guides on BJD dyeing you can find online for this, but it's really not much of a hassle. The bigger question is if buying the dye and setting everything up for it is not a lot of work for a pair of 1/6 ears.

After dyeing and wiping off any excess dye, you can seal and process like in the basic route.

 

Dying them could give you a nice effect because it is never even and acts a natural shading. BUT if you not dying a whole doll that’s a lot of work. You are probs better just to seal them up and hand paint them to the right color. Your best method for the “hair” look will be to dry brush with a rough boars hair brush, a thick plastic hair brushes, or a really crusty beat up synthetic hair brush. Rabbits have pale inner ears you could just use pastels and “vein line” it. You can get crazy detailed with it but sometimes simple mimicking is better than exact realism.