Dye Trees Mod adds one color of tree for each dye. These trees spawn in the world, somewhat rarely, and can generate with any log material, either vanilla or from mods.
They behave and often look like oak trees, dropping saplings from 5% of leaves and apples from 0.5% of leaves.
However, they also have a chance to drop dye items from their leaves, allowing dyes to be farmed.
There is also a new biome (which can be disabled in the config), the Rainbow Forest.
The rainbow forest consists exclusively of these trees, with clusters of every color of tree.
Animals spawn at a much-increased rate in this biome, and sheep may take on any color at random.
These biomes also do not allow the spawning of dangerous mobs. The only hostile mob that may spawn at all are tiny slimes, which cannot deal damage. Slimes in the rainbow forest, like sheep, take on random dye colors, and will drop dye items if killed.