my guess would be copes gray treefrog.. they are native to nebraska and juveniles can be green like that... did you notice any yellow on the inside of its thigh? i found this on another site:
In general, the Cope's Gray Treefog tends to be smaller and have smoother skin than the Gray Treefrog, and it is often bright green when calling
this is a link to the pic.. looks like the ones in your picture i think..
http://www.chicagoherp.org/herps/pics/frog/xCopes1.jpg
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my pets: clown treefrogs, reed frogs, big eyed treefrogs, tiger leg monkey frogs, gray treefrogs, milk frogs, cuban treefrogs, whites tree frog, green treefrogs, squirrel treefrogs, blue webbed gliding treefrog, red eye treefrogs, chameleon treefrogs?, mossy treefrogs, dusky salamanders, tiger salamander, box turtles, mud turtle, map turtle, yellowbelly slider, florida softshell, two saltwater tanks, five cats, two chinchillas, and a boyfriend.