Contrast

Look at the bird in the picture. Small but still visible.

Olive-backed Sunbird (Female)

Now, look at the other green bird. Visible but harder to spot. One might even need to zoom in.

Pink neck green pigeon (Female)

Difference? One is the same colour as the leaves in the background (and foreground). 

Take the bird below. It looks better with grey and yellow feathers. It is also pretty close to me. Hence the ability to see the texture of the feathers.

Long-tailed Shrike

Compared to the brown bird below. The grass is not much of a contrast. But it is better than brown soil or leaves.

Scaly-breasted Munia

Silhouetted against the sky.

Oriental Dollarbird

Bright colour but still in the leaves. 

Black Naped Oriole

Colourful with the sky in the background. The word colourful needs to be qualified. At the distance I was at, all my eye could see was a bird shape blob.

Coppersmith Barbet