What is the color of the pen when writing? For example, when to use black, blue, black blue and red.

Black and red will be used most of the time. Black is mostly used in formal occasions, such as exams and signing contracts. You can also use it at ordinary times. Blue is not formally used, and can't be used in some formal occasions, but it can be used for writing in daily life, especially when taking notes. In order to highlight the difference, you can use blue in addition to red, and pens of other colors can also have similar effects. Dark blue is relatively rare. The blue juice in the practical pen is black and blue, which can be used for writing and typing. If it is red, teachers will use it more to correct homework and test papers. Students' words are more useful when taking notes. Some important and difficult points that should be paid attention to can be marked in red, while those for other purposes are less.