I do want to give one warning though. It is tempting to want to change the color to get a "terracotta" look, but it will just look like terracotta colored concrete. In my experience shades of gray look fine on colored concrete, but when you start changing the color significantly it often just looks cheap. If you can afford it, and you really want the look, I would suggest splurging for actually outdoor rated tile, pavers, or stone surfacing. (I am guessing you are talking about a patio or sidewalk surface). To pay the cost of finding a contractor that can do color additives well (espcially in a small applications) vs. having it tiled (or tiling it yourself)...it will probably be nearly a wash.
Just some thoughts...consult a professional in your area.
PS;If you are mixing the concrete yourself, you should be able to find any additives you need where ever you buy the concrete materials. If you are hiring someone to do, make sure they have done it before, check references, and if possible go see an example of thier work. Concrete can last a long time...
[This message has been edited by antitrader (edited September 03, 2001).]