Plant sap, or just sap, is fluid transported in xylem tubes or phloem cells of a plant. Xylem cells transport water and inorganic nutrients through the plant; phloem cells transport sugary fluids and other biological molecules.
Types of plant sap
A large number of animals, all of the single insect order Hemiptera, feed directly on phloem sap, and make it the primary component of their diet. Phloem sap is "nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, [but] it is consumed as the sole diet by a very restricted range of animals". Phloem sap is physiologically extreme in terms of animal digestion: it is thought that few animals take direct advantage of this because they lack adaptations to this diet.
Many more consume phloem sap by proxy, feeding on the honeydew (a sweet secretion) of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap. Others feed on the biomass of insects which have grown on direct ingestion of phloem sap.
Sap colour is different for different plants. e.g.:
Plant sap Facts for Kids. Kiddle Encyclopedia.