Flowers like this do not need bright colours or prominant petals. They have no scent or nectar. Their stamens hang out in the wind, dropping hundreds of thousands of light pollen grains into the air. The pollen is carried by the wind and hopefully may land on a stigma of the correct species. Trees that produce catkins are this kind of plant.
Grasses are also wind pollinated flowering plants.
The picture on the right shows a typical grass flower head.