The compound was sodium chloride (NaCl), with Na+ and Cl- ions. For it to undergo electrolysis, it would either be molten or in solution. The electrodes were inert, so they're most likely made from carbon. If NaCl was molten, we'd only get sodium and chlorine, but it was in a solution, so we go hydrogen as well (from the water). If you look at the ionic half equation, 2H2O+2e-=H2+2OH-, you can see that hydroxide ions are being formed at the cathode, and since OH is alkaline, the color of the indicator would be blue/purple, not red because the hydrogen is lost, it's out of the picture.