I have seen Commodore 64 programs that have a line number followed by a colon. What purpose does the colon serve?
Most Microsoft BASICS allow you to put a colon as the first character in a line, and this has no effect on the running of the program (except to slow execution a bit). The superfluous colon is often used to merely insert a visual gap in the program listing, since you can't store a blank program line. Since many BASICs delete any leading spaces after a line number, the colon is also used to indent lines for increased readability, since spaces after a colon are preserved.