Using color eases the way to focus on a particular term that is important to the problem or exercise. Specially in the hint to elaborate the procedure of solving a problem using different color step by step helps explain things more clearly. It helps visualize different pattern in a problem. A simple example may be this [snapshot from KA content in kolibri.],
Visualization creates interactivity between the learner and the problem, which is really important for learner of grade 3, 4, 5.