You seem to be mistaken. Some operating systems display them as colour images, but that is not essential at all. They are mapped to Unicode characters, and so when displayed as characters, as in normal applications with an appropriate font, they are vector-based and monochrome. See e.g. http://www.unicode.org/charts/PDF/U1F300.pdf linked from http://en.wikipedia.org/wiki/Emoji
That's how the Japanese use them in their mobiles, no?