DLA (Diffusion-Limited Aggregation) is a growth model of bacterial colonies and tree-like crystals.
For details, see http://en.wikipedia.org/wiki/Diffusion-limited_aggregation
Rules are modified a bit here.
0a: Divide color-space into two sets, in the present case high-saturation colors C1 for solids and low saturation C2 for background.
0b: Place solid seeds randomly, colors chosen from C1.
1: Starting from random position, do random walk until the walker reaches any neighbor site of solid atom.
2: The walker is absorbed onto the solid. Choose a color which is close to the neighbor solid atom, from unused color in C1. Goto 1.
3: If no color is available, cancel absorption and continue random walk.
4: If no color is available for any absorption site, terminate the simulation.
5: Fill unoccupied pixels with unused colors.
Source code: http://d.hatena.ne.jp/ita/00010414/p1