Streak is the color of the mineral in powdered form. Streak shows true color of the mineral. In large form, trace minerals can change the color appearance of a mineral by reflecting the light in a certain way. Trace miners have little influence on the reflection of the small powdery particles of the streak.
The streak of metallic minerals tends to appear dark because the small particles of the streak absorb the light hitting them. Non-metallic particles tend to reflect most of the light so they appear lighter in color or almost white.
Because streak is a more accurate illustration of the mineral's color, streak is a more reliable property of minerals than color for identification.