I don't know the definition of tree miller...
[EDIT: as the video I linked to disappeared, thank Robotnik for pointing this out]
But there are a variety of semi-automated tree harvesters out there that are capable of a modest amount of AFK.
The idea is for you to have saplings in your hand, plant one, have a bone meal dispsenser adjacent, which bone meals the sapling.
Once the sapling grows into a tree, a large redstone device, which surrounds a 5x5 area pushes the logs out to a large wood collection area.
If you are daring, you can build an enclosure for a wither, and have the wither blow up the wood, most of which will get harvested.
Saplings are collected in hoppers at the bottom, and moved back to you, so you just have to sit and plant saplings.
There are many tutorials on the topics, in particular, google: Minecraft automatic tree farm (or minecraft AFK tree farm). A video from a more famous youtuber than previously posted is here:
You can add structures such as trees and grass (and others) using the Customize interface. As of 12W40A, you have to use More World Options.... From there, set the World Type to Superflat, then click Customize and then Presets. You'll be presented with a box that lets you share presets. The format for these presets can be found on the wiki.
To create a superflat world that also generates trees, flowers, and tall grass, use the following preset:
2;7,2x3,2;3;decoration
This will generate a world with a layer of bedrock, two layers of dirt above that, and then one layer of grass above that. Trees, flowers, tall grass, and so on will be generated. The trees will be fairly spaced apart.
Note that villages cannot be generated here, since they only generate in plains or desert biomes, and the Extreme Hills biome used to add the trees, flowers, and tall grass does not allow villages to spawn.
However, structures such as Abandoned Mineshafts and Strongholds can still be generated. Simply add ,mineshaft, ,stronghold, ,dungeon, etc. at the end of the preset. There are various parameters that can be added, such as ,mineshaft(chance=0.1). These parameters are detailed in the wiki.
If you want more trees, you can change the 3 to a 4 to use the Forest biome. This will generate trees far more densely.
Best Answer
Many colors of grass and trees come from biomes. Swamp biomes have darker, purple-y grass than plains, forests, etc. Likewise, the color of the trees' leaves come from this biome color data. If the grass isn't as purple as mycelium this is probably what you're seeing.
Unfortunately, this means that once in a biome, any grass is forced to that color, regardless of where the grass came from. This means that you can't dig up "swamp grass" and plant it in a forest or a jungle — it will just change color.
Sometimes the borders between biomes change rapidly and result in trees with different color halves and grass of different color seemingly in the same biome. This may be what you're seeing in your swamp.
Here is a jungle with a swatch of swamp on its border, showing the above effect: