Red: Used in areas with a higher risk of cross-contamination and spread of infection — most commonly for cleaning restroom areas including toilets and urinals
Yellow: Used for cleaning objects and surfaces in rooms and lower risk areas of restrooms including sinks
Green: Used in areas where food is handled and prepared
Medium/Dark Blue: General use, and patient rooms in hospital settings
White: Specialized use, including isolation rooms in healthcare.
Why separate cleaning materials into zones of use? Well, for the same reason you don't usually care to use a spoon after it has fallen on the floor, but you would probably go ahead and use it if it simply fell on a relatively clean countertop. By the same token, germs on a restroom toilet flush handle should probably not be brought directly to a sink in that room - or any room - by a cleaning cloth or sponge. Cleaning can improve sanitation, but sometimes it actually spreads germs around, bringing them to new food sources or moisture.
Cross-contamination is the process of bringing pathogens, those germs that can cause us illness, from one spot to another, and in particular a spot where it can infect. For example, a cloth used to wipe a patient's curtain in an ER can deposit germs if it is later used to clean a door knob. Then the germs can come in contact with hands, and an infection that would otherwise might never have moved out of the ER, ends up being carried to a sandwich and later being ingested.
In addition to ingestion through our mouths, we can also become infected by germs entering through our eyes, noses, ears, lungs and other "mucous membranes" or entrances into the body.
So the idea behind segregating germ zones is to reduce the chance of giving germs of one type a free ride into a zone they would not normally make it to. From a psychological standpoint, color coding also helps remind custodians and infection control personnel of the important job of breaking infection cycles that they perform.