Whether to assess the health of my plants, or for trimming while repotting, or just for aesthetics, I often want to know whether my orchids' roots are healthy or not. While I could easily distinguish a fat green root from a mushy black rotten one, I would often get stymied by thin dried out tan roots.
cattleya: fresh green root growing on right, tan mature roots on left
One very useful trick for making living roots to stand out is to soak them in water. Orchid roots are covered by a spongy layer of tissue called velamen. This layer protects the roots and helps them absorb atmospheric moisture. When dry, the velamen is a white/tan color, but it becomes transparent when saturated with water. This allows the inner green color of living roots to show through. New roots are always green because they haven't yet grown the velamen covering.
Phalaenopsis roots turn green after spraying with water
closeup: the transition from living root to dead end becomes evident
Angraecum leonis: the dead root turns dark brown, while the rest of the roots become greener
The effect on the angraecum leonis is less apparent because I watered it yesterday, so the roots have not had time to fully dry out. Root color is actually a way that many growers monitor when it's time to water a plant, since it's the most direct measure of how much moisture the roots are getting. This works best for orchids grown mounted or in clear pots where you can see their roots easily.
One caveat to this system: brown doesn't always mean dead. Some types of media can cause discoloration in the roots, and my paph orchids tend to always have dark brown roots. Plump sturdy roots are often healthy regardless of color.