Since this article was written, acid stains, which are indeed difficult to work with, have been largely supplanted in the decorative concrete world by concrete dyes. Dyes are, in contrast, quite easy and safe to use, and produce consistent, predictable, vivid color in a range that was not available in stains. They are available for outdoor or indoor applications, and will work on new or old concrete. Dyes can really help make a low-budget concrete feature like paths and patios look much more beautiful and harmonious with a garden!
They are also far superior to painting concrete in almost all cases. While all color will fade in UV light over time, concrete dyes are much more durable than paint. They penetrate the concrete and bond at a molecular level to it, rather than forming a film. This also means they are easier to reapply if you wish to touch up the color in later years, since you do not need to try to remove a peeling paint layer first, which is a serious PITA.
I recommend Ameripolish dyes, because they are available in sample sizes that are more suitable for small projects like this, and also make it affordable to test the colors before you apply them to a large area. The colors can be mixed to produce custom colors. They can be applied with brushes for small areas or sprayers for large areas. They can also be used with stencils to create decorative patterns or to mimic the look of bricks, pavers, stones, or even wood (experts only!). A new concrete installation can even be laid with a dimensional stencil and then dyed before the stencil is removed to create and very realistic illusion of brick or other pavers with mortar in between.
There are also integral dyes that are added directly to the concrete when it is mixed. These colors are very durable, but they can be a bit tricky. Since they change the composition of the concrete, they can affect its integrity, especially if you are trying to achieve a very intense color. I belive there are a few pre-made colored concrete mixes, which are probably the safest bet, since you know it has been formulated to work correctly, but of course that leaves your color choices limited. YMMV.
