There are several ways you can do something like this. If you just want to add some basic color highlights to a B&W photo, just pick a brush, set the size and color to what you want, and maybe lower the hardness and/or opacity a bit lower and then start painting.

Now, if you have a COLOR photo that you want to make look like a B&W image with some color in it, here's a really cool trick I picked up:

First, take your color image and make look completely B&W: click on Image > Adjustments > Desaturate. This will remove all the color.

Now, select the HISTORY BRUSH from the tool palette. This will allow you to paint over your image as it USED TO BE (get it, history). Just pick a brush style and start painting over whatever you want to restore color to in your image.

This will create the kind of effect you see in movies or magazines where the picture is B&W but the apple the woman is eating is still red.