(sorry for no image! its bugging out) 

A huge chunk of the world is brown. In the standard 24 pack, there are colors for pale skin and black skin, but none for tan/brown skin. While there are boxes with tan in them and an entire skin color set, these are not the standard pack. For brown children to color themselves correctly, they'd have to beg their parents for a special set, which is often more expensive or can't always fit with their other supplies. In a classroom where they have to use the teachers supplies, unless the teacher bought a special set of crayons, they may not be able to color themselves accurately at all.

This issue can be easily solved. There are colors, such as the neon lime yellow called "green yellow" (not the light green one called "yellow green". I am referring to the more yellow one of the 2), that most children find ugly and would not mind being removed. Tan is a much more useful color, and allows for children to draw themselves and see people that look like themselves drawn. It is also more commonly used in art in general.

On top of being good for brown children, it also benefits others who are not brown, as many school assignments require you to color people who are tan. If tan is made more accessible to the general public and classrooms, brown people will be colored more accurately. It will also bring more attention to the need for representation.

While I myself am white, I live in an area with a high brown population, and have noticed that their skin color was missing from the standard set. I demand a change, for our brown folk who need lots more representation than they get. If we can add white skin to the 24 crayon box, we should have no issues adding tan to the palette as well.