Say for instance my opponent casts Feat of reststance on his attacker and names the colour green to stop my blocker, Surrak Dragonclaw dealing any damage.
Does this resolve as expected or are tri colour cards not affected by protection from any singular colour in their cost (RUG for Surrak, for example), or can you name any of three colours in his cost and apply that accordingly?
Any feedback would be appreciated!
Yes, this works. A multicolored card is each of its colors, so {Surrak Dragonclaw} is a green card, a blue card, and a red card.
If something has protection from a color (green in your example), all it checks/cares for is a simple "is this relevant" yes or no kind of check. Is Surrak green? Yes, he sure is! Protection applies