Using Color's alpha channel with value greater than one provides odd result

The alpha channel isn't automatically truncated because some uses may require values outside of a [0,1] range, even though that may not be required here. As for why the colour changes as the alpha moves outside of the expected range, you would need to look at the blending operation in use that is giving you transparency in the first place.

In the case of a general 'fade' transparency, the blending operation would be;

 Blend SrcAlpha OneMinusSrcAlpha

What this translates to behind the scenes is;

 // Where;
 // - SourceColour: the colour of our object
 // - SourceAlpha: the alpha of our object
 // - DestinationColour: the colour of the scene behind our object
 (SourceColour * SourceAlpha) + (DestinationColour * (1 - SourceAlpha))

 // or alternatively
 Lerp (SourceColour, DestinationColour, SourceAlpha)

If you have a value between [0,1] you can see that the colour resulting from the blend would lie somewhere between the two inputs. However if the alpha has a value outside of that range, then an imbalance in colour would be introduced and you would start extrapolating beyond the input colours.