Thanks for playing! :)
I've used mostly grayscale because I wanted to use brighter colors to emphasize things, like the red eyes of the cultists or the golden aura / sparkles around the egg of light + I wanted to try myself at making grayscale pixelart.
I forgot to mention it on the game's page that you have to push the cast button on the UI to actually cast the spell because ENTER is not working. I couldn't spend that much time with development as much I wanted so I was in a bit of a rush at the end leaving some annoying bugs like this.
Altough I'm not sure what exact spell you tried. fire / fireball / fireblast (not available on level 1, tho its just a stronger meele "fire" spell) should work.