...there aren't really any true white/opaque inks out there in the market.
The pigments of white just don't hold well due to the "ink" consistency.
...too watery/thin. Has to be more paint based if you want to have that white
bling on your pieces. You can use paint pen but can soak up the under color
some times which can result in a tinted white of that underlying color. You
can go old school and use white-out but if you got no control, it ends up sloppy.
You can use white gouache or acrylic but, if you don't have that control, it can get sloppy.
or ...you can use a white gel pen, I sometimes use a brand called "milky" but you'll have to
experiment with different brands to see which fit your desired effect.
or you can use a white mean streak but, it takes forever to dry and if you don't have any control,
it gets sloppy.
or ...you can use a white crayon pencil, you'll have to experiment with brands to see which
will work for you. I use prisma white pencil.
did i just drop some science?