As far as casting, you can't tell the difference between a WF or DT if you're only going to be casting 10 to 15 ft.
You might even consider a 4wt line. First of all, let me make this perfectly clear, I usually advocate staying with the manufacturer's suggested line recommendations or, in the case of the heavier weights 6-9, underlining by one weight depending on the circumstances. Wait a minute… did I say underline? Yes I did. Macnair opened my eyes to this concept. You see, it's in the amount of line you aerialize
in making the cast. Consider for a moment that while you and I now know the weight in grains for the first 30-feet of our line, we don't know the weight of the remainder that typically includes the rest of the head, its rear taper, and the running line ... An interesting thought? It should be, because for every ten to fifteen feet we add to the initial 30 hanging in the air, we effectively add another line weight to the load carried by the rod. Said another way: a 5-weight line becomes the equivalent of a 6-weight when 40-feet of line are aerialized, a 7-weight with 60 to 65-feet in the air, etc. At some point, of course, our 5-weight rod will overload resulting in either a collapsed cast or worse, a broken rod. Then, as Doug would say, the Ancient Fishing Gods will be laughing.
OK, so what does this concept of underling a 7wt fly rod have with a small stream fly line? Plenty… the same concept applies but in reverse. Let’s say you’re using a 3wt rod with a 3wt line on a small stream. Most of your casts are going to be less than 15 or 20 feet. Take away the leader and you only have about 10 feet or so of fly line aerialized. Well that’s not enough line weight to allow the rod to perform as designed. So, in this case, its makes perfect since to overline your rod. Ten feet of a 4wt line might give you enough aerialized weight to allow the rod to perform better and thus improve your casting. So think about it. You may want to overline your new rod depending on how far you think you’ll be casting.
Line color has been a debated topic for years. Do a forum seach for "fly line color" and see what you come up with. If you cast well and your presentation is proper... it probably doesn't matter what color you use. Its not as much as the color of the line as it is the shadow cast by the line on the water. That's what usually spooks the trout. Yet having said all of this... I use an olive fly line on small streams. I also wear earthy colored clothing and branish no flashy accessories.
I use the Cortland 444 Clear Creek
fly line in olive. As I said, I'm not sure if the color matters though.
Hope that helps