Mixing with another shader:

![enter image description here][1]

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)

![enter image description here][2]

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.

![enter image description here][3]

Seen here with a .3 mix factor:

![enter image description here][4]


  [1]: https://i.sstatic.net/pNTCr.png
  [2]: https://i.sstatic.net/xlFK8.png
  [3]: https://i.sstatic.net/ONASO.png
  [4]: https://i.sstatic.net/6WBMk.png