Why streak beats colour
Surface colour depends on trace impurities, weathering and coatings. Quartz alone appears colourless, purple, pink, brown, yellow and black without any change in composition worth naming.
Streak is the colour of the powdered mineral, and powdering destroys the optical effects that make body colour unreliable.
The method
Drag the specimen firmly across an unglazed porcelain tile and look at the powder left behind. Use daylight rather than a warm bulb.
Hematite is the classic case: specimens range from silver-grey to black to earthy red, and every one of them leaves a red-brown streak.
Where it fails
A streak plate has a hardness near 6.5, so anything harder will scratch the plate instead of leaving powder. Quartz, topaz and corundum all defeat the test.
For those, a white streak is the expected non-result and tells you almost nothing beyond what the failure itself implies about hardness.
Building the habit
Keep the tile with the specimens rather than in a drawer elsewhere, because a test you have to fetch equipment for is a test you will skip. A single unglazed tile lasts years and can be cleaned with fine abrasive paper when it becomes crowded.
Record the streak colour on the catalogue entry even when it is the expected one. A file of confirmed results is what lets you notice, later, that a specimen you assumed was hematite has been leaving the wrong colour all along.