if you plan to make something that will be washed, as i did, then use a dye that is as permanent as possiible. i bought my dye at
http://www.dharmatrading.com/html/eng/1137-AA.shtml
and the color lasted a long time. i used it for kid's clothing and it got washed a lot. if you carefully follow the links, they give you a good idea of what you need.
if you're planning to make, say, a wallhanging, that might only get vacuumed, i would use rit and go cheap. why not? but i would not use as much water as they recommend. in the final rinse, you would lose a lot of color anyway, and you could keep rinsing until you're happy.