The alpha value of light's color property does not pass into shaders regardless of rendering path (deferred/forward). It is fixed for trunk but we also need a PR for 5.6.

Steps for reproduction:
1) Open the project attach below or create a new scene
2) Change the alpha value of the light color (i.e. Directional light) in the inspector
3) When light color is checked in the frame debugger, the alpha is always 1

Notes:
- It is fixed in here: https://ono.unity3d.com/unity/unity/changeset/71843a101959 and we need the backport to 5.6
- There are popular assets in AssetStore (i.e. Alloy Physical Shader Framework) and users who rely on this feature, therefore user prevalence is high
- Regression introduced in 5.6.0b2
- Image from the Frame Debugger is attached below