LR: How to Paint color back into Black and white? how do i do this?

I am using lightroom 2.2 I am trying to modify a picture. I want to turn picture into black and white and then "Paint" color back in on certain spots. I know i can do this in photoshop with layers and all that. I Know there is a way to do this in Lightoom. This is what i tried.

I desaturated picture then used an Adjustment brush (with saturation at +100, i thought this would counter act the -100 saturation i did on whole picture)

This did not work. Everything stays black and white. If i turn saturation back up, globally on whole picture, then the certain spot becomes over saturated. So i know that Adjustment brush is working.

I tried to click on black and white button (in HSL/Color/black and white panel) then resaturate that. Nada.

How can this be accomplished in lightroom?

Think outside the box. Try this. Instead of trying to paint color in, instead, paint it out!

Select the adjustment brush, and set the saturation on the brush to 0. Now paint everything that you want to be in black and white, and save the other parts for what you want to be in color.

Variation on BBD's advice: Brush over the entire image and then use Erase to remove the mask from where you want color.

You can also use an adjustment brush WITH color, and paint that on. You can do this even after you convert to B&W, and will be able to paint certain objects a colour. Mind you, this is one colour only, and not the original image's colour.

thanks for the advice. I never thought to do it the other way. It is odd that the way i want to do it did not work. but to paint everything in black and white will work. Some sort of bug?