So as to create such sort of effect, you need to separate targeted area from the rest of your footage.
Since you are on CS5, you may use awesome Roto Brush Tool (see this help section with two video tutorials included how to use it).
The tool you use depends on the shot you want to apply the effect to. The more planning that you do the easier the process. The most common tool is the Leave Color tool. When you want to isolate two colors you need to do some masking. If the color overlaps you have to do some rotoscoping. Plan the shot when only one item in the scene that has the color you want to keep and it's very easy.
I'd suggest you start with the Leave Color effect. The more saturation you have in the shot and the fewer similar colors the easier it will be. Take a look at this example and the settings: The shot is split so you can see before and after. If you look closely at the grass in the upper left corner of the shot you can see a little bleed. There is also a tree in the far upper right corner that shows some color specs. This could be fixed with a duplicate layer with the tolerances turned up and a rough mask to reveal the part of the shot that works perfectly.