The yellow one is definitely more active and is always chasing the grey one trying to feel but never seen it the other way around which is another reason I assumed male and female. The grey one is definitely more shyIf you've never kept honey gouramis, it can be tricky. I can't tell the difference with the red ones. Even yellows are not 100% from photos. But after years with honey gouramis (natural and yellow) I can usually spot males and females in a shop tank just by their behaviour.
Lots of places will say to go by their fins but even when I have the fish in front of me and they have spawned so I know which is the male and which is the female, I still can't see enough difference in the fins to be able to say that's definitely a male and that a female.
@Aimee16 Look at their behaviour as well. Males tend to be the bright perky ones, always on the look out. Females tend to swim around calmly, mainly on the look out for food.
How do yours interact? If one occasionally swims in front of the other, upright with its nose pointing upwards, that's a male displaying to a female. Just feeling each other with their 'feelers' could be either. One fish chasing the other could be a male chasing a female or a dominant male chasing a subdominant male.