Innodem main here, the fire explosion is from the perk stunning brutality, it stuns/ opens alerted enemies nearby to finishers, the explosion itself does no damage.
You are hitting several enemies on ground finishers because ground finishers have the same range as normal swing,its basically just a swing at an awkward angle, try putting all range mods on the innodem,it will be even easier to notice it, since its one of the longest weapons in the game
Edit: the fire explosion from stunning brutality has fixed colors, there is no way to change it