Sorting by colour in the wash
Short answer: Three piles are enough: white, light and dark. If you are unsure about a garment, it belongs in the dark pile.
The three piles
White on its own, so it can be treated with oxygen bleach. Light and pastel on their own, where dye transfer shows most clearly. Dark on their own, where released dye does no harm. That is enough for the great majority of households.
What breaks the system
New dark clothes belong in their own load for the first few times, whatever the pile. Terry also belongs on its own, because lint is a different problem from colour. See bamboo clothing for women.
When colour is not enough
Sort by fabric type too when it matters. Heavy towels and jeans wear on thin bamboo fabric in the same drum. See bamboo towels and bamboo underwear for women.
Frequently asked questions
Can navy and black be washed together
Yes. They both belong in the dark pile.
What about grey
Light grey belongs with light, dark grey with dark. If in doubt, choose dark.
Are three piles not too many
For one person two can be enough, meaning white and everything else.