You're missing a closing brace for olive. Should be:
"{olive}[VSH 2]{default} %s"
(╯ ͡° ͜ʖ ͡°)╯︵ /(.□ . \)
I made a small reproduction plugin and fat-fingered that.
With fixed code:
Spoiler
PHP Code:
#include <colorlib>
public void OnPluginStart()
{
char strAnnounce[256];
strcopy(strAnnounce, sizeof(strAnnounce), "You can set your {red}Boss Difficulty{default} by typing {red}/difficulty{default}.");
CPrintToChatAll("{olive}[VSH 2]{default} %s", strAnnounce);
}