What is the difference between warm and cold cloud seeding?

Warm cloud seeding stimulates precipitation production in clouds that are not cold enough to produce ice. Hygroscopic materials are introduced to encourage the formation of larger cloud droplets, which leads to the development of rain through the collision and coalescence process. Cold cloud seeding works by stimulating ice formation in clouds cold enough to develop it. This accelerates the growth of precipitation-sized particles.

Wiki Definition;
"Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed' somewhat, by an increase in volume, stickiness, or other physical characteristic of the material, as water molecules become 'suspended' between the material's molecules in the process. While some similar forces are at work here, it is different from capillary attraction, a process where glass or other 'solid' substances attract water, but are not changed in the process (for example, water molecules becoming suspended between the glass molecules)."