The appeal of Spore Frog comes in the form of its activated ability, which allows it to be sacrificed at instant speed to provide a fog effect , preventing all combat damage that would be dealt in a given turn
It targets things in the graveyard so it won't kill them with Horobi, Death's Wail (as they are already dead when the target happens, and just become attached when they enter the battlefield
It does everything you want, gets you everything you need, and has a low price to do so
It rewards good life management and smart timing , and its rarely a dead card when creatures dominate the table
The only time I'd cast from hand is if I needed to cast to keep from discarding or I needed something like a Vindicate to answer a massive problem that my foe has without the answer in my graveyard
That said, some colorless cards require you to spend specifically colorless mana to cast them, represented as a grey mana pip with a diamond symbol