My favourite style of pen, as it produces a very solid line.

The case colour usually denotes the ink colour, and since I like the purple pens I just empty them and refill them with universal inkjet printer black ink.

Depending on how heavily you use the pen, you can generally refill it several times before the tip finally starts wearing down too far and produces a much thicker line.

The pen brand name may vary depending on your country.
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https://www.youtube.com/watch?v=3jsjrIgD560