Try playing the game a large number of times (e.g. $30$ or more) with a friend and record who wins each time. You could do this at school and ask members of your class to play five games in pairs, then gather all the results together. Did someone seem to win more often?
If the first cube drawn out of the bag is purple, what is the
chance that the next one is the same? Or different?
What about if the first cube is yellow?