Finishes such as coating or plating is often applied to fasteners for various reasons. The most common reason is for rust protection. Other reasons include: appearance, reducing friction, prevent galling, ext.

Finish Corrosion Resistance Method of Finish Appearance
Plain NONE N/A Dark gray or dull black. May have light oil residue.
Black Phosphate POOR Conversion Coating Dull grayish black
Black Oxide FAIR Conversion Coating Black, sometimes with oil topcoat
Nickel GOOD Electrically Bright shiny silver
Chrome GOOD Electrically Bright, lustrous silver
Zinc GOOD Electrically Shiny silver. Can be iridescent.
Zinc w/ Dichromate VERY GOOD Electrically Often shiny yellow, sometimes other colors.
Mechanical Galvanized VERY GOOD Cold-Welded Dull gray
Cadmium (Cad) VERY GOOD Electrically Bright silver/gray
Hot-Dip Galvanized EXCELLENT Zinc Immersion Dull gray - Can have rough textured appearance
PTFE EXCELLENT Dip Spin Most commonly blue in color, however there are several colors available