As Bridgette and Shawna said, they can see more color/types of color than we can. Apart from health reasons, that's another important reason why we must provide extra and proper lighting for our birds - without it we deprive them of a good portion of their sight, because they need more and different light than we do in order to see in color. Lack of proper lighting is like putting a kind of blindfold on them.
For sure! They do see colours.
From what I have read! They see another part of the colour spectrum (more of the colour spectrum) than us. Or see more colours than us.
And that could be why they can tell the sex of another bird. It is also why they should have full spectrum light available to them.