Females in the genus Paramastax can be told apart from males by their drabber appearance and their often swallen abdomen, ended in a scissor-like appendage with which they are presumed to dig into plant stalks to spawn their eggs.
The above two photos show a male.
Paramastax rosenbergi is distinquised from Paramastax poecilosoma and Paramastax duquei by a different tale (only with the males).
The last reddish segment of the tail of P. poecilosoma has a characteristic protrusion (in Dutch: uitsteeksel) (with P. duquei it is smaller) while the last reddish segment of the tail of P. rosenbergi has an indentation (in Dutch: inkeping).
In females, the tails of most species look very similar.
1st photo: Montezuma Lodge area, Parque Nacional Tatamá, Risaraldas, Colombia, 28 June 2015
2nd photo: El Km 18 Road, Cali area, Valle del Cauca, Colombia, 12 June 2016