A very common “Effect” that photographers love to do is where they take a picture of a flower, and remove all of the color from the background… leaving the flower itself the only thing left in color. This is effect is called “selective coloring” and there are many ways to approach this, but in this tutorial I will explain a quick and dirty way to accomplish this in Lightroom 4.
I will presume you already know how to import photos in to Lightroom, as well how to access the develop module. So start with your photo already in Lightroom, and with the develop module open.
This is the image I will be working with:

Remove ALL Color first, with Desaturation Brush
The first thing you want to do is desaturate your ENTIRE image. Desaturation is the process removing color from your image. Since most of your image is going to be Black and White, it is much easier to remove color from the entire image, and then paint the color back on after the fact.
To desaturate your image you need to use an adjustment brush.
The adjustment brush tool looks like this:

When you click on it a new panel will open up:

You will notice there is a slider on there for “Saturation”. You want to drag that slider all the way to the left until it says -100. Then you take your brush and paint across the entire image. You may want to increase your brush size to make this go faster. You can increase your brush size by push the RIGHT bracket key: ]
As you paint across the image, the color will be removed. When you are done with this step, your image should be Black and White, like this:

Add Color Back to the Subject
Now that you have removed Color, you can quickly paint it back on. First resize your brush to make it smaller. You can do this by pushing the LEFT bracket key: [
Next, hold the ALT key on your keyboard, and then paint your brush across the areas you want to show color. Holding the ALT key “Reverses” the brush to make it do the opposite effect that is assigned to the brush. So since your adjustment brush is set to DE-SATURATE, by holding ALT while painting you will be RE-SATURATING your image.
This is what my final product looks like:

This entire process can be completed in about 5 minutes.