There are a few things to consider here.
1. Synthetic fibers (polyester) take dye quite a bit lighter than cotton fibers. I think the term "slightly lighter" might be a dissapointment to you once it's all said and done. Synthetic fibers are "man made" and could have been created a bazillion different ways. (one of the reasons to really dry clean things when the care label says dry clean only).
2. What color are the jeans now? and what color are you going to dye them? This can have a big impact on the outcome.
3. Stretch (which is what the polyester is most likely used for in your jeans ) is typically in the weft or fill yarns (these yarns go from right to left when your looking at your jeans). The reason I tell you this is that those will probably be the yarns that take the dye lighter and are not as visable from the surface of the jeans. Ever had your jeans unravel at the bottom... you will notice that in some pants those yarns are white but you would have never known had they not come unraveled.
4. I would suggest that you DO NOT use Rit dye. Not that it's bad but see if you can find a "better brand". I don't have a brand to suggest but I'm sure you could check around and find something of a more professional quality. ... unless your just messing around
(Rit Dye will fade as it is washed and I'm sure some other brands will too so be careful what you wash it with).
I hope this didn't get too long and drawn out... just wanted you to know what your getting into and what to possibly expect!