As some of you probably know, I've been saying for a while now that Marceline is no vampire, that she is just some shapeshifter who really likes old movies and cosplaying. Why else would she shout, "Blah blah!" when she was trying to be menacing?!
Marceline is not a vampire, just a half demon, shapeshifting, cosplayer. 02:10, November 13, 2012 (UTC)
The above image is my proof, in a world where there is no magic running rampant due to the lack of the nuclear radiation, a poor half demon girl never finds whatever gives her those shapeshifting powers.
It has been stated that Marceline has issues with her image and that is why she never looks the same in each episode, taking on an entirely new self is definitely what someone would do who has issues with their image and shapeshifting capabilities.
It makes just as much sense for her to be just a cosplaying shapeshifter with identity issues as it does for her being an actual vampire.
In the Farmworld timeline she would not have spent as much time with Simon as she did in the regular timeline, it is possible that she watched those classic movies with Simon and kept her cosplay to help remember him with.
Depression is something strong, when trying to hold onto lost loved ones one could do a lot of things that seem silly.
With the ability to shapeshift you can give yourself the ability to drink colors. You can't prove she can't eat anything other than red. She can shapeshift herself to appear to burn in the sun. You don't know those guys were dead, they could have been undead already, or they could have been raised by a spell, vampires don't raise the dead anyways, they make undead from the currently living.
Besides she doesn't need permission to enter someone's home and she is able to cross bodies of water, that means she isn't a vampire.