a classmate examines a substance that conducts electricity and is shiny but has no color. He says that is must be silver. What is his error?

He is making a conclusion from a single observation.

Silver isn't the only metal that is shiny and has no colour.

For example, lithium, sodium, magnesium, and aluminium all have about the same colour as silver.

Not all materils that conduct electricity are silver.

Explanation:

The conductive materials are many and different. The best known are metals such as silver, gold, copper, aluminum.

But some aqueous solutions of salts are also conductive, even the human body is conductive.

As you can imagine, not all of these materials or solutions are silver or even shiny. So the mistake is in concluding that all conductive material must be silver and shiny, when that is not always the case.