Here's a review of some white gel pens, white markers, paint and ink I've used for my watercolour sketches.
These are great for adding highlights, especially drawing into tiny areas where it would be extremely difficult to do so with a watercolour brush.
These are very convenient. For pens and markers, you can just uncap and use.
There's a huge variety of products that use white ink or paint so you can choose the right one for your art.
The one important thing to note is you should already make sure your art is dry before you use the white pens or markers.
White gel pen
My favourite white gel pen is the Sakura Gelly Roll which is available with line widths 0.3, 0.4, 0.5, 0.8 and 1.0mm.
The pen uses a roller ball and ink flow is good. Ink flow is better than the Uniball UM153 white gel pen that also have. There were several occasions where the Uniball pen clogged before I could use up the ink. Maybe I'm just more lucky with the Sakura Gelly Roll which I've been using for years.
Sometimes I use a white correction pen too. But I'm not sure what sort of pigment's used. It's important that the white does not turn yellow in the future. So far, the white from the Sakura pens have remained white over the years.
I use white gel pens for drawing and adding details. For covering larger areas, I use white gouache instead. The brand of the gouache doesn't really matter. Gouache is not 100% opaque and once you add a bit of water, you'll make it more transparent. Titanium White is more opaque than Zinc White.
If you want 100% or total opacity, consider acrylic gouache instead. But you have to get a dedicate brush for this because acrylic if not cleaned properly can harden and damage brushes.
White ink in bottles can be applied with dip pens or brushes. If the pigment is tiny enough, such as Dr Ph Martin's Pen White, the white ink can also be used in technical pens. Generally speaking if there's no mention of usage in pens, never use the inks in pens so as to prevent clogging.
White ink may be easier to use compared to gouache as the ink is already in concentration and in liquid form.
The ink bottle almost always have to be shaken before use to mix the pigments with the binder thoroughly.
That white block is from the ArtGraf set. This works just like solid watercolour or solid gouache.
The advantage here is you don't have to squeeze white paint out. Just add water and you can pick up the paint instantly.
Posca markers are some of my favourite opaque markers. Posca has a good variety of colour, ink flow is good and paint is quite opaque. The markers are also available in variety of tips, even for white.
I use Posca markers for drawing thicker lines that the white gel pens can't.