Hi rploetz, I'd say that hardwood in general (naturally colored, without any installation patterns like herringbone etc) is very much in style right now. There's a huge resurgence of the use of natural materials, and exposed wood/wood grain.
In terms of color, I'd do what is most functional for you. For a small den, I'd recommend doing a lighter color wood, since it'll really open up the feel of a space. But if you have kids, pets, or anything else that tends to scuff up/scratch floors, you might want to consider a darker/redder wood.

Thank you so much!!! I have both kids and pets, so I'll be going with the darker wood.

I see Walnut, a medium dark tone with a lot of grain. Choose a color close to your pets fur color- yellow lab on dark wood? You'll see every hair and scratch.

We made the flooring pictured in this post. It is natural-colored Ipe from Brazil, hand-finished with satin sheen. The installation is near Poipu on the south coast of Kauai, Hawaii. Very popular amongst our customers nowadays is anything "cerused" - the whitewashed look on oak. In a light color, and with a UV-cured oil finish, it stands up to kids and pets, and looks great.