Chemicals for electrochromic reaction ?
Easiest way is with an iodide, e.g. $\ce{NaI}$ or $\ce{KI}$, and starch. 

An old trick was to sprinkle a bit of $\ce{KI}$ on a (damp) slice of potato, stick a couple of wires into the potato and attach a small cell to the wires. Iodine is liberated at the positive terminal and it reacts with starch in the potato to make a deep blue color.

There are more suggestions from Flinn Scientific, such as using an indicator to show acid or alkali liberated at electrodes.