You need to use the correct spell type against the correct enemy type, kind of like rock paper scissors. For example: red hurts green, green hurts blue, blue hurts red.
I ran out of time on my last day to create a tutorial or instructions screen so I put some instructions in the itch page below the game. Next time I will definitely put more time aside for a tutorial or instructions for everything to be more clear. Thanks for the feedback, I appreciate it!