Ok, just wanting to see if someone has an answer...
I have a {standard bearer} in play, ability is that if an opponant plays a spell or ability that could target a flag bearer, they have to target a flagbearer in play.
Also in play on my side is a {mother of runes} which can give any creature protection from the color of my choice.
They play a spell, it is on the stack, then I activate the {mother of runes}, giving the bearer protection from that color spell, would the spell then fizzle?
This would work!
Assuming the spell targets, it would have to target your Bearer. After gaining protection, it is no longer a valid target, and the spell on the stack will fizzle.