The difference between those two in particular is an ether on the phenyl group on JWH-250 which is not seen on JWH-018. Metabolism tries to make things more water-soluble, so if that oxygen in the ether of JWH-250 is acted upon it would likely be demethylated to a hydroxyl. It's possible that JWH-018 could have an hydroxyl added at that position, but I think it's unlikely.
An alternative possibility is for the carbonyl to be cleaved, resulting in the potential for the common segments of JWH-018 and JWH-250 to yield common metabolites.
I can't say anything for certain without knowing what specific metabolites are tested for in the Redwood test. And a generalization to all indole synthetic cannabinoids regarding possible metabolites cannot be made. Each compound would need to be considered individually.
Given the structural similarities between the indole synthetic cannabinoids it is possible that some may share similar metabolites.