HHC is probably illegal in Colorado because the state specifically prohibits “chemically modified” THC isomers, such as hemp-derived delta 8 and 10. Since HHC also comes from hemp-derived CBD, it is possibly illegal in Colorado by definition.
Most US states don’t have any laws covering HHC. Despite this compound being around for decades, it seems lawmakers haven’t heard of it yet. Because of this, HHC is legal in most but not all US states. Here’s a snapshot of the current state of HHC legality in the United States.