"Streak" is the word used to describe the color of a diamond when it is reduced to powder form. It has nothing do do with an actual streak.
Although a diamond's streak is typically white in hue, it can manifest itself in a host of different colors. For example, an industrial diamond’s streak is usually black in color, but it can take on other colors, including red, orange, yellow, green, blue or brown. It may also be colorless. A diamond is the hardest natural substance on earth, and its hardness can cut through a surface as opposed to leaving some sort of residual streak behind.