Chemicals for electrochromic reaction ? I would like to detect the electric current going through the water solution using the colour reaction in this water solution. What chemicals can be used for this solution to make it electrochromic.
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.