1. Set into Turbo Mode -Shift+Click the Green Flag (this isn't necessary, but is much faster) 2. Pick your clay types -Press 1-9 and Q-I to add a corresponding color from one list to the other. 3. Pick Sizes -Change X & Y to the dimensions of your area. 4. Randomize! -Press the Space Bar to randomize the blocks! 5. Place it down -Go into Minecraft and place down the blocks as they were randomized!
The reason the colors tend to form in horizontal lines has to do with the way the "item <random> of <Clay Types>" function works. I obviously do not own Minecraft. Microsoft and Mojang do.