it's probability that the two shapes are red+ that they are green + that they are pink. so (1/6)²+(1/3)²+(1/2)²

you mean 0.5 ? the probability of seeing a pink side when you roll the die n times at least twice is 1-(probability of seeing 1 or 0 pink side when you roll the die n times).

probability of seeing 1 pink side when you roll the die n times is 6*(5/6)^(n-1) (Binomial Distribution, or just basic combinatorics).

probability of seeing 0 pink side when you roll the die n times is (5/6)^n (you have 5/6 chances to not have a pink side at each roll)

so you study the function to see when it becomes greater than 0.5