Stains / discoloration

This can be caused by

- Water from spills
- water from continual source leading to mildew (black) or decay (brown/white) or alkali (white) or bleeding up of adhesive
- Urine (dark) from pets or wet diapers
- Improper maintenance with water or harsh chemicals
- Traffic pattern wear
- Excessive harsh sunlight (wood looks starved near South or West facing windows)
- Light deprivation under area rugs, large low furniture
- Oil soap residue

Obviously you can minimize discolouration by correcting the issues causing the problems and perform correct maintenance of your hardwood flooring. For dark stains you can lightly abrade surface with fine sandpaper, feather out area, dampen cloth with 50/50 household bleach & water and lay on stain for 30 minutes, remove, let dry, re-color if necessary. For waxed floors, clean with renovator or paint thinner (combustible), re-wax clean and buff. If all fails, screen and coat, sand and refinish, replace severe boards.