Lighting and shadow in threejs

I use directional light with color 0xffffff to light my scene, but the result is a little bit dark , see below img
so, this is my first question, why it would be dark ?

and then , I add ambient light with 0xffffff color, the intensity set to 1, it is default value, the scene become bright,
but the shadow 's color is nomore black, it becomes the ground color. and the ground color becoms more white, see below img
so, the second question is, why this happens ?

this is the origin ground color texture:

I tried the same thing in unity, and the result is the same

the last question is, how to make the scene a little bright, but do not influence the ground color and make the shadow color always be black color?

edit: I have posted all imgs in one page

update: I found another issue, the shaodw is not complete, this is the shadow in three.js, this is shadow in unity,
it seems like the bottom surface of the box is not cast shadow , if I rotate box around x 180 degree, up side down, then the shadow look fine, so I suspect the bottom surface has something wrong, but I can’t tell what it is, maybe front /back face related ?

I have fund the reason
I make the intensity of directional light to 0.8, and add an ambient light with intensity 0.5, so the scene is bright, the reason why shadow color is the same as ground color, is because it is this color, the shading math do the stuff.
And one more thing I found, when an model is loaded in three.js and unity, the material color is default to rgb(150, 150, 150), so the color of box is not bright, I after I change the color, it looks good.

The last issue about shader shape is that, the bottom surface have wrong winding, so the surface’s front face is wrong, after change it, the shadow becoms normal.