What type of snake is bright green on top and bright yellow underneath?

If you saw one it depends where you live, but it could be one of the following:

green tree snake: http://las.new-England.net.au/wp-content/uploads/2007/03/images20070304-035-treeSnake-sml.jpg

http://www.tvwc.org/AssetsGreenTreeSnake.jpg

green tree python: http://www.kingsnake.com/ihs/I.H.S.%202005%20PHOTO%20CONTESTHeat%20pits%20of%20the%20green%20tree%20python%20-%20Michael%20D.%20Kern%20-%20IN%20CAPTIVITY,%20SM.jpg

rough green snake: http://www.wilddelaware.com/wp-contentrough-green-snake-11832.jpg

natal green snake: http://api.ning.com/files/L1ySGyMwLZ3hID057bBHbPN6Eqw8ML160RN4P6rDPxM_NatalGreenSnake.jpg

Asian green snake: http://www.dekayi.info/R_I_P_%20Green%20snake_filestree2.jpg

Hope this helps.