the nightshade will darken it a fair bit, specially if you put quite a lot of it on...if it's turned out too dark and you don't like it, you
can paint over it with the base colour again...try to leave the dark shade in all the crevices though, if you can...now, because you'll be painting over quite a dark colour, you will probably need to give it 2-3 nice thin coats (as Duncan often says in the tutorials, add a little water to your paint on the pallet to thin it out a bit, if you use the paint straight from the pot, it will be too thick, and you'll probably gunk up all the crevices and details, and possibly end up with thick "lumpy" paint on the model)
...apply one thin coat of the base colour - it will probably look a bit "streaky", that's OK...just let it dry, give it 15-30 mins...then apply another thin coat...if it's still a bit streaky, give it another, repeat as necessary (it is important to let each coat properly dry though! - if you try to paint over a half-dry previous coat, there's a good chance your brush will shift the previous coat, leaving a dark spot, with a ring of thick lumpy paint - I've done that many times, and it sucks, so trust me, be patient!)
painting takes time and patience, and learning a lot of small techniques through practice, there's no need to rush, and nobody expects your first models to look spectacular! you're learning

- I've been painting for 2 years (which is
nothing compared to some folks around here!), and looking back at my fist painted models...yeah, they're not the best!
just give it time, keep trying, you'll get the hang of it
oh, also, if you add a bit of water, or "lahmian medium" to the nightshade, say, a half-half mix, it won't be as dark...if you want a more subtle effect