I can definitely recommend a fine asian gel pen (kind of rollerball). It is precise and very easy to use, unlike many other tools I've tried so far.
It's a perfect affordable tool for detailed drawings, like miniatures, doodling, pattern fillings, i.e. things that require precise,
fine strokes and points. Of course one can draw anything, but bigger areas will take some time to fill them out.
0.5 mm is the most common stroke width for handwriting.
For serious work a lot of ink is needed, so I personally use and recommend pens with standard-sized mass-produced refills.
Pens: for a start go for something like Muji or Muji clone:
This image above shows Muji clone - these are compatible with standard asian refills (13 cm). Original Muji are maybe better quality, but I am not sure about refills' compatibility--- I think original ones require some special sized original refills.
Note: I don't recommend or not recommend this brand, it is just an example for quicker understanding of choice aspects.
You can buy all such thing online e.g. on Aliexpress. Search for e.g. "gel pen refills". Beware that the quality may vary. Buying small amounts by different sellers is a good strategy to find good quality product.
There are 2 most common asian refill sizes, below is a photo for reference.
The longer one at the bottom (~13 cm) is the most common standard asian refill and it can have needle tip or bullet tip. Needle tipped pens can have finer stroke--- 0.38 mm, but that depends, you must test to see.
These 13 cm refills are always available by various sellers on Aliexpress, and can be found in different colors.
The shorter one (~11 cm) has a bigger volume, lasts longer, but I have seen them only with black/red/blue ink, 0.5-0.7 mm stroke.
Also I believe this type is more likely to be found in Europe & US shops.
It might have different design, e.g. come in a metal case:
Ink consistency & permanence
Higher price and known brands might be better in this regard, but anyway, always test it before buying big amounts.
Possible problems: bad ink consistency or ball defects. Too fluid ink is hard to work with especially for detailed drawings. This can result in daubs, uneven strokes, etc. Too viscous ink is also bad, because you will need to apply pressure to draw, and that is even worse
than too fluid ink--- pens with too fluid ink can be at least used to roughly fill big areas. Good test is to make dots on a paper - just gently touch and release the tip on the surface - fine, equally sized dots should appear.
Most inks are permanent, but if you want to preserve the drawing, protect it from water and mechanical damage.
Finding good colors is a real problem.
Usually there are 3 colors in mass production: black, blue, red. Green, violet, magenta are quite common too. Obviously black is the most demanded for drawing.
Other colors are more rare/expensive. On Aliexpress colored pens/refills are sold in bundles, normally without specifec color choice.
For specific colors--- better try contacting the seller and ask.
For my work, I had problems finding good natural colors, like specific shades of brown and green.
It is also feasable to inject ink into the tube with a syringe - there are many good color inks sold in bottles for fountain pens, but I am not sure how it will work out. Currently I am planning some experiments with tube refilling, so maybe I'll update this answer in a while.