Painting... New Walls.

I would definitely use a primer--it's a must over new plaster. If your contractor did put up white primer, you should be fine with two coats of a finish color unless your finish color is really dark in which case you may want to use a coat of tinted primer. I agree with previous commenters to buy a good quality paint, I have had good success with the Behr brand at Home Depot and the Lowes store brand. Also, for walls that aren't in the kitchen or bathroom i prefer an eggshell finish to semi gloss. Eggshell is a bit shinier than flat but not at all shiny.

Good luck!

Another tidbit of advice--if you are doing dark, bold colors you are much better off spending a little extra cash on slightly more expensive paint than going with a cheaper brand.  We did a lot of dark colors and used Ralph Lauren paint from Home Depot--it was about $5 more per gallon than the other paints they had, but we didn't need a dark colored primer--just one coat of contractor primer and two coats of the good stuff and it looks fantastic.  You'll save yourself money in the long run and a lot of time. 

A lot depends on the type of paint you buy and how dark you are going.  I assume your walls will have a coat of contractors white primer on them already.  This undercoat covers seals your  texture if you put some on and hides the tape and plaster seams.  I would suggest not to skimp on paint.  Cheap paint requires more coats so there is no money savings in the long run.  It is also thinner and splatters more on the brush and roller.  Do be sure to check out the "oops" table if your hardware store has one.  They are loaded with paint returned usually because the customer didn't like the color but if you check the brands you might get some awesome paint for $5 or under .  Since your walls are new and white you won't need any other primer.  There are mold inhibiter additives that you might want to check out if you are painting in an area that will be moist.   Also, decide what gloss level you want.  Be sure to use washable enamel in bathrooms and kitchen.  I made the mistake of painting one wall in my living room with a  flat finish and it just looks terrible and got dusty!  I never had to dust any of my walls.  It was so much better in a semi gloss finish.  Can't wait to see the pictures of your new color.