Understanding the meaning and vital importance of 'complementary but not matching' is probably the most difficult part of wearing a pocket square. I'm not sure my own grasp of colour is really up to the task, so I tend to go with 'whatever I happen to think looks nice'.
If I were to post-rationalise this particular outfit choice, I might point out that the pink in the pocket square picks out the pink shirt, while the blue flowers are the same shade as the blue overcheck on the jacket. However, all I really care about is that I found a pocket square with little elephants on, to go with my Oriental Club tie.
Sometimes, that's all that matters.
(Pocket Square is from Thomas Pink, Tie by Dege & Skinner)