I am thinking is more of a male female deal.
Male Andinoacara Rivilatus (Green Terror) have bright markings in the unpaired fins. This is your light colored one.
This is a male I had as an example
The darker one is female as it has no bright markings in the unpaired fins.
A female I had as an example
The females belly looks to be rather round possibly full of eggs. She may be trying to present to him that she is ready.