Avoiding use of a red pen in providing handwritten comments on student work
In the past I have also received suggestions about marking in red, and had various discussions on the topic. My only reason for continuing to mark using red is that it contrasts strongly with the typical blue or black of the submissions.

It is often cited/claimed that red is an "angry" colour, or students find it demoralising, and so it should be avoided. But that doesn't really follow when the markings are "ticks" or other constructive affirmations.

Using blue pen when the work is blue or black does not provide a good contrast and corrections are more easily overlooked. (I have used blue pen for marking when, after having had students ask if it matters what colour they use to write, I have received work written in shades of pink. In that case, blue gave a better contrast than red.)

Green has been suggested as an alternative, but then there are studies to indicate that information written in green is not easily remembered, and if the reason for writing corrections is to help the student learn, green would then be counterproductive.

Other colours such as purple and black also do not give a good contrast. And you may well find that the students who complain now that they don't like red will later complain that you didn't show them the correct answer/method/result because the new colour used didn't stand out from the rest of the work.

There is also the issue of marking individual items or questions and then totalling the marks across multiple pages. The individual marks need to be easily seen by you so that you don't inadvertently miscount and record a lower mark or grade for the student work.