OK this may sound totally redneck, but I have done it & it is pretty awesome.
Take a steel wool scouring pad & put it in a bucket of vinegar for about 24-48 hours. The steel wool will dissolve in the vinegar. Then you take a mop (you will have to throw this mop away when you are finished) and mop this solution onto your prepared concrete floors. You can mop on as much or as little as you want, but the finished product is like a terra cotta looking finish.
You will have to mix multiple steel wool pads with multiple gallons of vinegar dependant on how big of an area you wish to stain.
You can go over it once, let it dry & reapply as many times as you wish. The more you put on the darker it will be.
It will not be an even finish as you can imagine sort of rustic... well a rust look (you are essentially rusting the steel wool with the vinegar - which creates the rust colored stain).
You will have to apply a clear sealer & may have to do that every few years.
The outcome is really very neat & all for very inexpensive.
Let me know if you have any questions... this may not be the look you are going for, but I thought that I would post it as a do-it-yourself alternative.