[Riddy]

You gotta use a format for that, then use SendClientMessage(playerid, COLOR, string);


EDIT:

I'll help you :P...

new string[no. of strings];
format(string, sizeof(string), "%s has not been found, please use /help or /commands to check for the command list", commandname);

(You can do the command name part your self)

then

return SendClientMessage(playerid, 0xFFFFFFAA, string); <- try that... not sure if it works or not, just a guess