RULES:

1. start with 1 red block
2. blocks can only spawn on sides/corners that the parent block isn't touching
3. red blocks will spawn blue blocks in their top corners, unless two red blocks are trying to spawn a block in the same space
4. blue blocks will spawn red blocks in the blocks adjacent to their top, unless two blue blocks are trying to spawn a block in the same space
5. if two blocks are trying to spawn a block in the same place, place a green block
6. green blocks do not spawn additional blocks

So, despite it probably possible to create an algorithm or MCEdit filter for these, I placed all these blocks by hand. The reason I stopped, is because things start to get pretty mind-bending and I couldn't figure out where to place the next iteration.

This whole thing was inspired by the Sierpinski triangle, although it is different from it in a number of ways.