Those two states are asking the Supreme Court to overthrow Colorado's law (passed by referendum) legalizing recreational marijuana use, as being in conflict with federal law.
It's amusing to see a pair of conservative "states rights" states asking SCOTUS to use the supremacy clause to overthrow a state law based on a federal law itself justified by what conservatives would consider an expansive interpretation of the commerce clause.
That last bit may be why they are suing Colorado rather than suing the Feds to uphold the existing law.
Originally shared by +The Denver Post:
BREAKING: Nebraska and Oklahoma sue Colorado over marijuana legalization
Nebraska and Oklahoma sue Colorado over marijuana legalization
Nebraska and Oklahoma ask U.S. Supreme Court to halt marijuana legalization in Colorado
To a SCOTUS geek like me, an interesting aspect to this is it may be an "original jurisdiction" case. Because it's between states, SCOTUS might well be the original court for it, and not act as an appellate court.