Can you stain a lighter stain on a white pine log a darker color at a later date? We are in the process of building a log cabin.
Probably not if you have finished the logs with some type of sealer such as a varnish or polyurethane.
If not totally sealed you can with stain/finish products that basically paint on. This type of finish would be very similar to modern types of exterior semi-translucent water based deck stain/finish.
You can always go darker. As Michael eludes to you will have to either remove the sealer or use a burn technique we have used for grills and fences that we have built. This is basically scorching the surface to give a more natural darker appearance.
I don't understand why you wouldn't do this during the build phase though as it has to be way easier to do vs wait until everything is done and work around an occupied house.