It shows pink by default, but it doesn't fit with my server colour scheme.

http://prntscr.com/nw7sbj

I think you can change the color in the lang file

bonus.happyhour.name=Happy Hour

to

bonus.happyhour.name=§aHappy Hour

?

I tried, but it seems hardcoded.

You should be able to overrule this

return ChatColor.LIGHT_PURPLE + MobHunting.getInstance().getMessages().getString("bonus.happyhour.name");

Well.. it didn't work when I tried, that's odd.

bonus.happyhour.name=§bHappy Hour

worked fine on my test server