Hello,

Here's my understanding:

The color of the pencil's exterior (pencil's body) is the same as its tip (the part that you use to color).

corps des crayons = body of pencils
sont assortis= matches
à leur mine = to their lead (lead color)

In marketing, a catchy way to say this in English: See the pencil, know its color. In other words, when you see the color of the pencil's body, you know the color of its lead (what is used to color)

I hope this helps.