YELLOW SKULL

[Tobaias.]

Guys, why i have a yellow skull on Sunny Meadows map? This appear only when i start a solo... Anyone?

[TheNightglow]

the skull is supposed to indicate that you can earn a trophy on sunny meadows in solo
if you keep an eye on difficulties, especially custom high ones you will notice the same skull in 3 different colors (bronce, silver and gold)
the color changes depending on the difficulty multiplier (i think gold was >15x)

if you play sunny meadows in solo with a skull difficulty you can unlock a bronce silver or gold trophy

[Stupid]

Because that's the map you can do the Apocalypse Challenges on, which is only available when playing on singleplayer.

[Radiate88]

Yeah, the apocalypse challenge requires you to correctly figure out the ghost, get all 3 optional objectives and get a photo of the ghost. The bronze trophy is at 6x multiplier or higher, the silver one is 10x and the gold trophy is 15x. There's some "recommended" difficulties that fall under the requirements, but if you use a custom difficulty, you can adjust the settings to your own likes.