Contact support
Contact support but honestly, you save 400k gold by using the 2015 gem
You should be able to rank from the item storage shouldn’t you?
Go to item store, select the gem and use it.
You have to contact support, say you want to use the 2016 gem on colossus and don't have the option. They'll adjust your inventory afterwards.Not a good idea though. Better to save the gold and rank up another 2016 champ too. Are there two other 2015 mutants you'd use a gem on after Colossus? Probably not.