Why isn't a mineral's color a reliable way to determine it's identity?

The correct answer to this question is that minerals often have different colors. While a metal like gold will always look like its namesake and be shiny, there are minerals like Calcite and Quartz that come in different colors. There are many elements which can affect a mineral's color. These elements can include radiation, light, and heat. An example is the mineral Proustite, which turns dark when it around the light.

Another aspect that can affect the color is tarnishing. Many elements like silver and copper tarnish over time, changing their color. There are also many minerals that have the same color, which makes it hard to use color as an identifier.

Minerals differ in colors although most of them have a specific color range.

However, some minerals have alternate color when they are grounded to powder than their natural attribute. This is the more reason why Geologists and other mineral researchers make use of tiles of unglazed porcelain usually referred to as a ‘Streak plate’ to discriminate minerals by their streaks. In another hand, they may have to scratch the mineral over a surface to get the real color as some amount of powder color is deserted when this is done.

Nonetheless, making using of streaks is a more dependable way of ascertaining a mineral identity than colors. Despite the fact that the mineral examples are diverse in colors, it leaves a rosy dark colored streak when scratched over a streak plate.