Pulse of Llanowar not effecting what is castable.

I had [[Pulse of Llanowar]] and seven basic lands onfield (but no swamp), and [[Darigaaz, the Igniter]] in hand, but it was not castable. I tapped one land and selected black, but it was still not castable, so I tapped another two lands selecting green and red, and it still wasn't castable. I tapped another three lands, this time selecting black,red,green again. Finally, now with six mana in my pool (including BRG), the card in hand was finally able to be put on the stack.