Here is my approach. I believe that it is the easiest to understand. The student should answer the question in the way that makes the most sense.
There are 4 red candies and 3 green candies to choose from. No other candies are mentioned, so I will make the assumption that we are choosing just from these 7 candies.
We need to determine what is the probability of choosing 2 red ones. We also need to determine the probability of choosing 2 green ones. We then need to sum these two individual probabilities to get our answer.
2 red: P(1st one is red) = 4/7 followed by P(2nd one is red) = 3/6. The probability for this to occur is 12/42
2 green: P(1st one is green) = 3/7 followed by P(2nd one is green) = 2/6. The probability for this to occur is 6/42
Finally, the sum f these two is 18/42 which can be reduced to 3/7.
Thus, the probability that they will "match" is 3/7.
Therefore, the probability that you will get 2 distinct colors is 4/7. (1 - 3/7)