Here is something that I thought many of you would find useful, how to color things to look like wood. This is for a piece of cut wood, not the trunk of a tree, so think of how you could apply this technique to a wooden fence, old signpost, wooden crate, etc.
The nice thing about wood is that Copics come in so many brown colors it's easy to find the tone you want for the type of wood you're looking for. I'm not as familiar with the different families of wood, so I can't give you too many recommendations, but I know that there are colors like Redwood E19, Walnut E59, and Light Mahogany E07. These give you a good reference for your darkest colors to make up that wood type.
1. Start with your lightest brown. In this case I'm going with E31. I don't have to color evenly, but if I have streaks I want them to be in the direction of the wood grain. Sometimes I purposefully leave a bit of white just to make it lighter.
2. Streak in your middle brown. I'm using E34 and I go in the direction of the wood grain, leaving plenty of areas where you can see the lighter color underneath. Again, it doesn't have to be perfect. You'll want some of the woodgrain thick, some thin. Make some of it crooked (not too crooked).
3. Lightly add your darkest brown. Each time you go a shade darker use less of that color so it doesn't overpower the lighter colors. Notice how little E29 I use. This makes it easier for the next part.
4. With your lightest color go back between dark wood grains and add light back in. Look closely at the diagram to see what I mean. Each stroke of E31 doesn't quite touch the stroke next to it, but it comes pretty close. On small areas this is easier to do with the Copic fine nib or the flat edge of the chisel end of either type of marker.
Make these strokes pretty juicy, since this pushes the darker colors out to the edge of each streak, but washes the levels together so your individual streaks become a piece of wood. The trick is to keep these irregular so the board doesn't look all the same.
Now look at the last step, see how all the colors blend together and each of the dark streaks doesn't stand out so much. This looks like a good, simple piece of wood.
If I wanted to add a knot to the wood you can do it in the same way. Make the knot from your middle and darkest color, then when you are streaking in your light color bend around the knot, just like real wood grain bends around a knot. You can make the wood as simple or as complicated as you feel. I once has a picture with wood floors and a wooden table that I worked on the wood grain for a couple hours. I really didn't need to spend that long, but I wanted it to look "perfect".
For faded wood:
Instead of using my lightest brown, use the colorless blender. This will give you that sun-bleached look like driftwood.
For weathered wood:
Add a layer of gray. Here I took my finished piece and just streaked in W3 and W5 to give it that old gray look you get from wood that's been sitting out too long. Remember, I choose Warm grays because they have elements of brown in them to begin with, perfect for graying out old wood.
This final piece is one that I made a few years ago for the Oregon Asian Celebration. It was part of a larger poster of shoes from around Asia for their 2007 theme "Footprints of Asia". Notice how even though I had already colored the wood I could layer in the soft colors of the bamboo and yet you can still see the wood underneath. The fine lines accenting the bamboo were drawn with a gray multiliner. I can't remember what colors I used, but this was on Color Laser Copier paper.