Just to add to what Ptarmic Wumpus has said...
I dye both cotton and silk with fiber reactive dyes. Silk takes the dye very well and vividly for a majority of colors. Keep in mind that FR were formulated for cotton so the colors you see as examples are for cotton. Like I said, silk dyes spot on for most colors. I can get an o.k. black but browns and sage have been difficult for me on silk. They tend to color separate, meaning the component colors that make up the dye "come out".
For smaller fabric swatches i.e. fat quarters a method called Low Water Immersion works well. I have a tutorial on my blog... http://desertdyeworks.blogspot.com/2009/07/something-fun-and-quick.html
Have fun and be prepared to get addicted to it!