R.M.
Nope, if it's too dark, they probably won't be able to fix it, but you can always ask them to try.
I am a longtime painter, so I know a bit about this subject. First, unless you are using acrylic (water-based) paint over oil, there is NO NEED to prime first. Most high-quality paints will cover old paint, unless you are painting a light color over something like navy blue, in which case you might need to prime it. But Behr, which is at Home Depot, has a paint line that includes primer in the paint, so if there is a color you like in that line, that's an option that will save you a step.
BUT - here's the thing: Most paint jobs require two coats, and a coat in a normal room takes one gallon, so you will need two gallons for the room. So if the two colors are similar, you could use the paint you have for the first coat, and then buy a second gallon in a lighter color. Two tones actually usually looks better on a wall, however, painting the darker version over the light version usually works better than the other way around.
Have I already told you more about painting than you ever wanted to know?
ALWAYS paint a few sample swatches on the wall before committing to buying a color. Some brands have mini sample sizes you can buy, and some places rent cans so you can try out a color before spending $35 dollars or more. Good luck.
Also good point that Jane made about it being deceptive over primer. She's right -- once the whole room is painted, it will look lighter than that first section looks, though it still might not be what you wanted. Truth is, you never entirely know until the whole room is painted if it's exactly what you wanted.