Species within the genus Eurema (Grass Yellows) range from Asia, Africa, Australia, and Oceania, to the Americas. 
The Grass Yellows are generally weak flyers, fluttering low around bushes and flowers.
At least 71 species are currently known. 
There are 5 species within the genus Eurema in Japan: Eurema blanda, Eurema brigitta, Eurema hecabe, Eurema mandarina and Eurema laeta.
The main differences between the species are:a) the number of black spots on the underside of the forewing, combined with
b) the shape of the (apical) black markings on the upperside of the forewing and
c) the shape of the apical area markings on the underside of the forewing.

Eurema hecabe is generally distinguished from other members of the genus by the very irregular inner margin of the dark apical band on the upperside of the forewing (evenly rounded in most other species). 
Eurema hecabe (sometimes also called Common Grass Yellow) looks very similar to the Eurema blanda (sometimes also called Three-spot Grass Yellow).
The difference, however, is on the underside of the forewing.
In Eurema blanda there are three small spots and a fourth bigger one in the middle of the wing; in Eurema hecabe there are two small spots and one bigger spot in the middle of the wing.

Eurema hecabe also closely resembles Eurema mandarina.
While Eurema mandarina is distributed primarily in Japan, Eurema hecabe is widely distributed in Asia, Africa and Australia.
Until recently, both species were considered to belong to the same species with mandarina as a subspecies of Eurema hecabe.
Specific genetic research showed that individuals with a yellowish forewing edge indicated it would be Eurema mandarina and individuals with a brownish forewing edge would indicate it would be Eurema hecabe.
However, this was not conclusive and as a result it is impossible to distinguish the two species from a photo.
For now, however, I consider the ones that I've seen in Japan as Eurema hecabe.

Yoyogi Park, Tokyo, Japan, 22 September 2018