buttons opacity (or alpha)

Hello everyone!
I would like to make the buttons i’ve created using GUI.Button slowly appear. I thought about changing the opacity value during time, but there’s no opacity (or alpha) value inside the GUI.Button method. Can someone give me an idea about how can i achieve this effect?

Use:

GUI.color.a = valueFrom0To1;

just before drawing the GUI Button.

This will also work for all other GUI elements, not just buttons.