Nice question and probably the mindset would be different of all person to solve the mathematical problem.

According to me, we can solve it by two means one if we need to know the probability of selecting at least one from all the four sweets we will go for a probability solution that is by the method of permutation.

Second one we can solve it normally like we have a total of 80 sweets, 20 from each color If we choose the sweets blindfolded it may happen that we will choose all blue in first 20 pickups, then 20 all red in other 20 selections and afterwards the 20 that might all be yellow and in these 60 collection of sweets blindfolded contains no green sweet but if we collect single more there would be no chance of missing the sweet from any of the colour. Therefore my answer will be 61 so that i will be fully confident for selecting the sweets from all the colours.

Hope the answer will be helpful