hi,
if you take a look at the textures and compare them to other textures of my creations, you will see that they simply are not dark at all. And now I think I understand better what you mean, but what you describe is indeed more a problem of the renderer, having problems to spread the light inside of an object.
And thats something that happens with many renderers and thats the reason why I adopted a linear workflow long ago and in the meantime linear workflow is even included in many packages.
Now back to the spruce. As example I attached a rendering of one of the spruce trees, carefull, slightly exagerated ;-)
It´s standard render settings and also standard camera exposure settings.
I loaded the plant and simply set the diffuse color to "2". This takes just some seconds to do and you can see that you get a bright green. You can do the same to Translucency, you even can crank it up to 100 so that the outer parts of the tree start to glow. Those higher values help a bit to spread the light further, but in general there´s not much I can do about that. And thats also not something that only happens on the spruce pack, it happens on every object that has such dense foliage.
Sometimes it can help to use the transluceny texture inside the luminous slot, only with a low value.
But those are settings that for sure will brake most other scenes and so I dont use them.
So if you have your scene ready and only this object is to dark, then you can change the according setting. But I simply can´t offer a setting that works for all people on all scenes. Thats not possible at all.