I want to make a narrow concrete, decorative column like the ones found in this garden..
What is used as a mold for these types of columns?
What gives the concrete those subtle color changes (the red and/or blue hues)?
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You make the pillars using any round type of tube or pre made form. They are made from wood,metal or plastic.
Depending on the finish- if you do not mind the crackled and holey look like in the small pillar on the left then you just pour the concrete into a form and leave it. If you want a smooth finish you need to use a "vibrating poker" which will remove air bubbles and separate the concrete from the aggregate making a smooth finish, like on the tall one on the right.
You can either dye the end product with some concrete stain or use a dye that you mix into the concrete it self. Either will give you various effects and they both last for many years.
The pics in your question look like columns made of blocks to me, rather than a monolithic pour. They also look very smooth surfaced. These types of blocks can be made by DIY, but not easily since they are most likely hollow and have guide holes for 3/8 or 1/2" rebar rods or alignment dogs. The rods are used to align the blocks as no mortar seems to be used between blocks. In making these type of blocks, the mix is high in cement and low in aggregate. There is a release agent used in the molds as well.
I rarely discourage DIY experimentation, but in this case I think you might be much better off finding a supplier of the blocks and perhaps coloring them yourself with an after pour dye. Getting the quality, internal detail and smooth finish will be difficult, frustrating and most likely cost you more to build than buying something at a masonry/stone supply.
The major home centers (Home Depot, Lowes, etc) sell round cardboard forms for pouring round concrete columns. They're near the Sakrete and wire mesh, etc.