This study investigated student difficulties reading equipotential diagrams and the effect a visual change to these diagrams had on students' ability to interpret these diagrams. Equipotential diagrams are often drawn with a uniform line thickness and color. We modified the equipotential diagrams to use color variation and line thickness to indicate the sign and strength of the potential. These changes, which are consistent with theories of visual attention and grounded cognition, exploit students' innate ability to perceive color and line thickness variations to communicate the electric potential. Students compared electric potentials between indicated points on given traditional or modified diagrams. Results show that these students, even after instruction, responded with a distance only electric potential rule. In addition, we found that the modified diagrams did not have the predicted positive effect on students' ability to rank electric potentials; we suspect this is because students' rule for electric potential did not account for charge sign.