Pot may be legal in Colorado, but that doesn’t mean the state has no illegal pot. Legal growers have illegally sold cannabis across state lines, and illegal growers can forge a paper trail to enter the legal market. It’s a problem for law enforcement, which needs to combat this gray market to keep the state’s $1.5 billion cannabis industry on the right side of federal compliance. Starting this summer, the state could be able to identify all aboveboard cannabis. Using technology previously employed to track premium American cotton from gin to shirt, growers spray their legal plants with DNA that acts like a molecule-size encrypted bar code.