It was probably the "Color" page at my site (glassattic.com) that you saw with info about Kato's Color Concentrates:http://www.glassattic.com/polymer/color.htm(oops... can't find it there but it should be, so I'll come back later when I find the exact page)
You may have also read about inexpensive ways to do polymer clay that I've written before here at Craftster (they have a lot of overlap):http://www.craftster.org/forum/index.php?topic=155163.msg1548395#msg1548395http://www.craftster.org/forum/index.php?topic=73664.msg1638472#msg1638472http://www.craftster.org/forum/index.php?topic=177119.msg1833985#msg1833985
...Or perhaps you were seeing this page:http://www.glassattic.com/polymer/kids_beginners.htm
(under the Info. for Beginners
category, which is a little different?)
Most clayers don't need the concentrates because it's so easy to mix colors just by mixing colored clays together** (though it helps a lot to start with very pure
colors, not those which have already been toned down in the package). Artists oil paints (in tubes) and alcohol inks also work very well as concentrated liquid pigments to color clay.
When you say that you're using "white" clay, you should know that if you're using bulk white Sculpey
(in a 2 lb or larger box), that line is very brittle after baking in any thin
or projecting parts so is seldom used by hobby clayers or "serious" clayers too. They're also fairly soft for good handling and details, etc. (flesh-colored SuperSculpey
and Sculpey III
are a bit better, but not as good as the other brands/lines, with Fimo Soft in that grouping being the least strong but still fairly strong.)
**All that's really needed
to make just about any color of clay is 3 "primary" colors (pure versions of a yellow, red, and blue... or even better Lemon Yellow, Fuschia, Turquoise), plus a little black and a lot of white. Translucent or mica-containing "colors" can be added for more effects, etc.
If you want to make the absolutely clearest colors you can (for tinting translucent clays, e.g.), then using a liquid concentrate would be better than adding a bit of solid clay because the liquid concentrates are generally transparent, but that's not generally necessary to do when mixing colors.
There's loads of info on how to mix just those few colors together to get virtually any color you want on the Colors page at my site linked to above.
As for how much it takes to color a certain volume and color of clay, it's probably not a lot since it is a concentrate (and not much oil paint or alcohol ink either), but that also depends on the total volume of clay you usually work with and you didn't mention that as well as the color saturation you want ... in other words, you'd use a lot less if you were making pendants and miniatures for example, than large sculpts or coverings for tabletops/cabinets/etc., a lot less if you don't want the most
I assume that the feel of the clay doesn't change much after adding a concentrated pigment because the amount of colorant is so small, and Kato clay is stiffer than the other brands except for FimoClassic anyway if you're using those. You probably wouldn't want Sculpey, SS, or S III, any softer than they already are though, if it did change a lot.