What is the purpose of the cardboard?

Question

In an experiment, the orange liquid obtained in (b) was used to dye an entire piece of cloth. A cardboard square was placed over a part of the cloth and the arrangement was left out in the sunlight FOR A LONG TIME.

Answer 1

Likely the cardboard square was used to block sunlight to determine whether sunlight caused the color to fade.  Possibly light photons could change the chemical structure of the subject fluid, especially it is was a dye.

Answer 2

block sunlight