I am doing a cow patty cake and they want the inside to look like a cow patty too. They want white cake. How can I make a white cake look gross when cut into?

Make the filling from ganache with some shredded wheat added.  Tell them it's "cow patty filling" and leave the cake plain white.

Out of my years of baking for weird people, I would say that making the whole inside of the cake that gross would not be an order that I would accept.  Cake is FOOD after all.

 Reminds me of a country western novelty song I heard once, by Jim Stafford:

http://www.madmusic.com/song_details.aspx?SongID=350

(And I think if I were called upon to make such a thing, I wouldn't be able to resist adding a figure of a cowgirl "on a moped she called Flame" ("Yippee-i-ay - Cow Patti!")

I actually saw Jim Stafford in concert singing this song. I've been telling my dh for years that I want a moped and I'm going to call it Flame.