If you want to add colors to the show_activity() functions, I suggest you use the cromchat include file -
https://forums.alliedmods.net/showthread.php?t=295046
After downloading the file, you can replace ALL chat messages of that type in just 2 simple steps:
1. Add #include <cromchat> in the plugin.
2. Add the color codes in the plugin's lang file.
And you're done.