Saying that they are "green" is not necessarily accurate. Even on prickly pears that you gather from the desert, there are different varieties. If you are shopping in a Hispanic (Mexican) grocer you will seem them called "Tuna". More specifically: "Tuna Roja" and "Tuna Verde" ... ie red tuna and green tuna. The green tuna, to me, tastes more like a true pear. It has a slightly off taste compared to a pear, but it is similar enough to a pear flavor that I understand why it got called a "prickly pear". The red variety is, however, probably the flavor you are looking for. It is distinct and like nothing I've ever tasted. Gamey watermelon is the best description I could possibly have. It's a decidedly "wild" flavor.
Locally I found a woman that would pick what ever was in her front yard for $5 a 5 gallon bucket. I picked up 3 or 4 of them, and juiced them. The red color made my back patio look like a set of Dexter - there was so much red juice and pulp flying around that it stained the patio concrete. I still have 3 or so gallons of juice frozen ... I'm thinking of making a prickly pear skeeter pee. The mead I made is aging - and I don't want to devote another batch of honey to something if I don't like it (or want to alter the recipe).