BBC - Aphids, see bicolored aphid photo, information and links to Wikipedia.
Biology Teaching Resources - aphids.
Blackman, R L, Aphids. Ginn Co, London, 1974. 175 pages, (part of Invertebrate Types series).
Dixon, A F G, Biology of Aphids. Arnold, London, 1973, 58 pages (part of Studies in Biology series, number 44).
(Both these books are excellent introductions to aphids but are sadly difficult to source outside libraries.)