The colons have been in earlier redis versions as a concept for storing namespaced data. In early versions redis supported only strings, if you wanted to store the email and the age of ‘bob’ you had to store it all as a string, so colons were used:
SET user:bob:email bob@example.com
SET user:bob:age 31
They had no special handling or performance characteristics in redis, the only purpose was namespacing the data to find it again. Nowadays you can use hashes to store most of the coloned keys:
HSET user:bob email bob@example.com
HSET user:bob age 31
You don’t have to name the hash “user:bob” we could name it “bob”, but namespacing it with the user-prefix we instantly know which information this hash should/could have.