Species within the genus Eurema range from Asia, Africa, Australia, and Oceania, to the Americas. 
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 spots in the underside forewing cell, combined with
b) the shape of the upperside forewing black markings and
c) the shape of the apical area markings on the forewing underside. 

Eurema hecabe (sometimes also called Common Grass Yellow) ooks 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; in Eurema hecabe there are two small spots and one bigger spot in the middle.

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