It is a brand name for a pastry that is heated in an electric toaster. They are popular with children.
In the case of Pop-Tart red lipstick, it is a play on words.
Pop=pop(popular) culture (or pop music)
Tart=sexually prosmiscuous young lady, cheap woman
Paris Hilton and Brittany Spears are two currently famous pop-tarts.
I answered your question about "Pop Tart Red" in your other thread (on "wouldn't be caught dead"). I'll repeat part of it here: it refers to the color of lipstick that a "tart" who sings "pop" music would wear. The first person who "popped" into my mind was Madonna!
To compound the word play, pop tarts very probably have artificial sweeteners and flavors. Artificial food coloring is another likelihood. Lots of falsehood in a snack! Appealing to the uneducated palate.