Lighting Rookie Question

Hi there,
I am thinking of creating a 3D game with day night cycles. My environment would be a forest. I am wondering if a simple adjustment in lighting will allow for the illusion of day and night. I am using basic, non pro. So it looks like I have no use of shadows.

Is going for this type of setup, a) viable by light adjustments (ie, I do not need to change models) b) doable with basic, non pro?

Not sure about day/night question, but it would need to be scripted. However, you can have shadows in Unity free.
I guess you would need a timer, so you can get some scripts here.http://wiki.unity3d.com/index.php/TimeOfDay

Thanks.
I am just trying to figure out, by switching to a day from a night, the trees for instance, would go from a blue to a free in color. This of course is caused by moon light Vs sun light.