The white belt is, by Corpora, reserved for Knights. That is the only Society-wide reserved belt color.
A tradition was started some time ago to use additional belt colors to indicate some SCA-specific relationships. This tradition appears to have started in the Pennsic kingdoms, and spread throughout the Known World from there. First was red belts for squires, then green belts for apprentices to Laurels, and yellow belts for proteges to Pelicans. Some kingdoms have made these formally reserved, by including a sumptuary law about them in their kingdom laws.
Caid does not reserve any belt color but white.
In Caid, a squire may be given a red belt by their knight, usually with the personal arms of the knight on the chape (the metal point of a scabbard, used here to indicate the tip of the belt.) However, anyone can wear a red belt.
An apprentice may be given a green belt by their Laurel, often with the personal arms of that Laurel on the chape. However, anyone can wear a green belt.
A protégé/protégée may be given a yellow belt by their Pelican, often with the personal arms of that Pelican on the chape. However, anyone can wear a yellow belt.
Please note that if you do wear a red, yellow, or green belt, someone might assume you are esquired, apprenticed, or proteged to a peer. For example, if you are a fighter who is not a squire, but wear a red belt, don't be surprised if you're not asked to become a squire -- the knights may think you are already one!
This does not mean you shouldn't wear belts in these colors; it just means that there may be a bit of communicating to be done to clarify that you are (or are not) in an SCA-specific relationship.
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