It is not often that a plant can through a visual color change indicate its need for water, however the aerial roots of tropical epiphytic orchids indeed do. Epiphytes are plants that grow on other plants or structures but are not parasitic (Photo 1).
Photo 1: Epiphytes on Tree
They derive their moisture and nutrients from air, rain and nearby debris. Most of the orchids used as house plants are tropical epiphytes.
The roots of these plants not only serve to anchor the plant to trees or stone, they also function as water storage units capturing water during rain events. The roots have a unique structure that enables them to absorb and store water. Phil Gates, a botanist at Durham University in the UK, has a blog entitled, Beyond the Human Eye - An insight into a microscopic world, invisible to the unaided human eye. He has sectioned and photographed an orchid root (Photo 2).
The xylem vessels that conduct water from the roots to the leaves consist of the ring of bright yellow cells at the bottom of the photograph. Surrounding the xylem vessels is a layer of blue packing cells. Exterior to the packing cells is a row of hexagonal cells beyond which are a layer of dead cells called the velamen layer. The velamen layer functions as a sponge soaking up water as the aerial roots are exposed to rain or mist. Interestingly the velamen layer changes color based on water content and is an excellent indicator of the plant's water status. Dry velamen reflects light and is white or silvery (Photo 3),
but when the velamen absorbs water the green tissue underneath becomes visible and the root takes on a green or mottled green color depending on the species (Photo 4).