I noticed that the scoreboard list commands do not have a colon at the end of the first line, similar to that which is used in list, banlist, and whitelist list.

Multi-line result commands that do not have a colon at the end of the first line:
/scoreboard players list
/scoreboard players list <player>
/scoreboard objectives list
/scoreboard teams list
/scoreboard teams list <team>

I think that adding the colon to the end of the first line of these would be a good formatting change that is consistent with other list commands.

Another reason I suggest this change is to make it easier for administrators implementing RCON clients to parse the output. Having the colon makes it easier to identify the end of the header and the start of the variable results. I can happily provide more details regarding this upon request.