This butterfly in my garden is like many of the others, hard to identify, but the closest match in my book and online seems to be Salome Yellow (Eurema salome) (link to butterfliesandmoths.org). The book was more helpful for this butterfly than this online site though other sites verified the single spot on top. While the bottom side of the wings is very similar to both the Cloudless Sulphur and Tailed Sulphur, the heavier border and single dark dot on top seals it for this species, I think! 🙂
See all my Costa Rica Butterflies & Moths GALLERIES.