Protection from

[Deathseide]

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?

[Keyeto]

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.