1-the probability to pick a certain ball of these 3 is:1/3

2-if after picking the ball, we were told that the ball contains the colour red (for exe.), then the probability of telling which ball it is becomes:1/2

(same thing for the colour green).

then why cant we by just picking the ball consider that it contains a certain colour and by that the probability of question 1 would become 1/2?