We can spot that Val\textquotesingle s guess and Elvis\textquotesingle{} guess are the same, so both of them must be wrong. Therefore, John is right. 
There is a ball in a box and three kids are guessing what colour it is.  Val says: "The ball is white."  John says: "The ball is blue."  Elvis says: "I agree with Val."  They open the box and find only one of them guessed right.  What colour is the ball? 

1) white 
2) blue 
3) Uncertain 