Mixing with another shader:

Using this node setup, the Mix factor is now your transparency slider, and the diffuse color is the color of the material. (in this case, white)

Note that using a diffuse shader as the other shader will shade the material slightly. If you want a solid color, you can use an Emission shader but only when seen directly from the camera.

To do this, you can mix it with a transparent shader with the IsCameraRay output in the Light Path node (Add node > Input > Light path node) as the mix factor.

Mixing with another shader:

Using this node setup, the Mix factor is now your transparency slider, and the diffuse color is the color of the material. (in this case, white)

Note that using a diffuse shader as the other shader shades the material slightly. If you want a solid color, you can use an Emission shader but only when seen directly from the camera.

To do this, you can mix it with a transparent shader with the IsCameraRay output in the Light Path node (Add node > Input > Light path node) as the mix factor.

Seen here with a .3 mix factor: