It's the first thing that happens with a dark-paged notebook: you pick up your usual pen, write a line and can barely see it. The pen isn't broken. Almost all the ink we use is made for white paper, and knowing why saves you from buying pens that won't work.
Why ordinary ink disappears
There are two ways to color an ink. Dye dissolves completely, sinks into the paper and lets light through, which means it's transparent. Pigment is particles that don't dissolve and sit on top of the paper, which is why it can be opaque. Ballpoint ink is a paste colored with dye, and fountain pen ink almost always is too, so on black or deep blue they leave little more than a shadow. Gel pens can carry far more pigment because the gel holds it in suspension, and that's why they exist in white. Sakura launched the first gel pen in Japan in 1984.
White gel pens
They're the foundation, and three names come up in almost every comparison. Sakura's white Gelly Roll comes in three tips, 05, 08 and 10, which leave lines of 0.3, 0.4 and 0.5 mm: the number is the size of the ball, not the line. The Uni-ball Signo Broad leaves a 0.6 mm line, very opaque, and in exchange dries slowly. The pastel Signo has a 0.7 mm tip and dries matte. The same thing works with all of them: light pressure and a slow stroke. Pressing harder doesn't make them whiter.
Silver, gold and pastel
Metallic gel pens are opaque, and people who have compared them say black is where they shine most. Pastels are useful for separating notes by color. Not every colored gel pen works: dark shades are hard to make out, and one brand can have a pink that shows and a purple that doesn't. You have to test them one by one.
Paint markers and chalk markers
These are for titles and fills. Poscas are water-based pigment paint, with tips from 0.7 mm up to several millimeters, and you shake them with the cap on and press the tip on scrap paper until the paint comes down. They dry in a couple of minutes, and once a layer is dry you can cover it with another. Chalk markers, which wipe off glass, are permanent on paper. In our tests, markers behave well on our paper, but each brand lays down a different amount of ink, so try them on the last page first.
Brush pens
This is where most of the confusion is. The brush pens most of us own use water-based dye ink, which is translucent, and black dulls transparent colors. The ones that cover are pigment-based: Tombow's white Fudenosuke, which goes on almost clear and turns opaque as it dries (the brand suggests two or three layers), Pentel's Milky Brush, or metallic brush pens.
Pencils
Graphite and charcoal don't show on black. Light colored pencils do, and whites differ from one another: soft, waxy leads cover more in a single pass, and harder oil-based leads keep a point better. On dark pages, a light layer of white underneath makes the color on top look brighter. For guidelines you'll erase later, use a white pencil with a light touch.
What doesn't work
Blue or black ballpoint, fountain pens with ordinary ink, highlighters, markers with translucent ink and graphite pencil. The line is there, but you can hardly tell it from the paper.
Five habits
Make a test page at the back of the notebook and note which pen made each line. Let ink dry before you rest your hand on it or close the notebook: a metallic gel pen takes about a minute, and fine tips take less than bold ones. Keep a loose sheet under your hand. Don't drop gel pens into a cup: Sakura warns that air bubbles form that way, and that storing them lying down or standing up makes no difference. And if one stops writing, scribble circles or tap the tip on a pad of paper, because heating or shaking it won't fix it.
The pen we use
Every Bocetta notebook comes with a 0.8 mm white gel pen in the box, with opaque ink. It's the same one we test our paper with. If you want a second one for your bag, we sell it separately. And if you don't have anywhere to use it yet, see the notebook.
Sources: Faber-Castell on pigment and dye inks, Gelly Roll white line widths, Sakura's FAQ, Posca's FAQ, Tombow on the white Fudenosuke and a comparison of white pencils on black card.