glee was really very, very cute indeed......

"How do you, as in the 'Glee' film, make the flowers in color and the rest black and white?"

You can use the Secondary Color Corrector.
First select the color you want to keep. Ticking the Show Mask checkbox can help doing this. Once you got the color selected tick the Invert Mask, untick Show Mask Only and then turn the saturation down to 0.

That sounds easy BUT if the thing you want to keep colored has any wrinkles or creases i.e. is not well lit then it becomes quite tricky and may require some tweaking e.g. rotoscope. Much easier to plan ahead and get the lighting right.

Of course that's just for one color. For different colored flowers repeat for each color. At a guess I'd say as they shot this on film and therefore had more color data to work with it would have been easier to pull off than if this was shot on DV.

The trick to doing this and other FXs such as chromakeying is if you run into trouble get the part that defines the edge of the subject working as well as you can. Ignore problems well away from what you're working on elsewhere in the frame. Other bits are relatively easy to fix with a mask.
In this case I'd duplicate the track and convert the bottom track to matching B&W. Then I'd go back to the top track and use a mask to get rid of anything else that the secondary CC missed. Also don't dispair if this starts to take a lot of time to get right. The canned demos always make it look simple.

Bob.

@Ulf:

http://www.jetdv.com/tts/TTS02-10.pdf