[crazygringo]

Yeah, it's the chalk dust. It's that simple.

Also different colors on whiteboards is occasionally helpful for clarity, e.g. color-coding an equation to a line in a graph. But that's pretty minor.

[lovich]

we had colored chalk for that purpose in my school for math at least

[hollandheese]

There is much less selection in colors for chalk. And many of them show very poorly on chalkboards. Hagoromo's green for example, is almost impossible to make out on our chalkboards.