If you want to be more specific/accurate, it could be a "plain brown corrugated cardboard box". Non-color words that could be used to describe it are "neutral" and "natural", since most people already understand them to usually be some shade of brown.

The main purpose of the Colour Box is to develop a child's visual sense of colour. Primary colours (Red, Yellow and Blue) are introduced in Colour Box 1, which is first presented to a child at 2.5 to 3 years of age. Colour Boxes 2 and 3 add more tablets to the mix, expanding a child's colour knowledge.

Color Box 3 is a large wooden box in the Sensorial Area, containing 63 color tablets: gradation sets of 7 tablets of each of the following colors, blue, red, yellow, green, orange, purple, grey (including white and black), brown, and pink. This material is presented after success with Color Box 2.

Brown kraft and white are the most common shipping box colors because they are the two most naturally occurring colors of corrugated board. Choosing any other color for a shipping box requires printing the color onto the corrugated board. Typically, this process involves printing the color onto mottled white cardboard.