Makes Color constructor public

Ok, I rethink about MonoGame and its compatibility with XNA. Now I wisely prefer extensions over core changes. 

This commit just unblock existing constructor. Why ? If you write extensions for Color on low level you must write something like this -

 Color c = new Color();
            c.PackedValue = 0xFFFFFFFF;
            return c;

This is inelegant and has lower performance than this

 return new Color(0xFFFFFFFF);