With this small cactus farm you get comfortably and automatically always enough cactus to produce green paint. The consumption of raw materials is minimal.
40x glass block
52x stone or other building material
Due to the thoughtfully placed fences, the cacti drop automatically as they grow and fall into the water flow. This transports them to the funnel and deposits them in the chest.
The whole thing works without redstone and with minimal material costs. The whole thing can be nicely integrated into villages by the many glass and the water movement.
Build a chest and then place a funnel on it from behind (sneak mode!).
To the left and to the right of the funnel there are 3 stones each. In the row behind there are only 7 stones, one row further there are 2 stones, sand, stone, sand, 2 stones.
Next row again only stones, then again the stone-sand combination and finally 2 rows of stones. Now the base area is already done.
In the next step, place two stones on each corner and put glass blocks everywhere in between, only the place above the funnel remains free (you can also take stones, etc., but that does not look so great. In no case use fence or glass panes because the dropped cacti can hang on them!)
If now everything looks like in the second picture we go on to the next step.
On the field between the 4 sand blocks, 2 blocks must now be placed on top of each other. Seen from the front, a fence part is now attached to the left and right of the upper block, then remove the two blocks again (VERY IMPORTANT!). Place cacti on the sand blocks and then it should look like on the third picture.
In the last step we put a glass block on the chest (in sneak mode) and then put at the very back left and right a bucket of water in each. The places are very important because of the current which then ALWAYS leads to the funnel and thus no drops are lost.
Now go out above the chest and place another glass block on top of the first one.
Tip for the cactus farm in the village: Leave a block free to the left and right of the chest, then pull up a 3 block wide and 3 block high wall away from the chest, roof on it, wooden door in and you have a new building for your villagers (more houses = more residents spawn!). My inhabitants in the 360 world love the house and use it more than the "original" houses. The farm itself also has a glass roof and cyan/turquoise glass blocks.
IMPORTANT FOR MINECRAFT-PE:
With my version 1.4.1 there is the problem that cactus drops on standing cacti dissolve into thin air. Here you can not solve the whole thing so nice with water cacti but must increase the sand, fences and walls by one each or set higher, how this looks is shown in the last picture. Thus, the drops do not stick to cacti and are transported neatly to the funnel.
Development platform: XBox360 (Screens+modification in PE on Android)