user6512: How is a movie made to show selective colors. For instance, in Pleasantville a sister and brother are transported to the universe of an old black and white TV show, and the movie switches to being B&W. Then, as the protagonists cause the inhabitants to have 'real' emotions, like fright, anger, or love, things (people, places, etc.) start having color. In the middle of the movie, many objects are still B&W, yet many more are now colorized.
Sin City also used this effect (the whole film was initially shot in full color, and was converted to black-and-white. Colorization is used on certain subjects in a scene), for example.
How is this effect achieved? Is it post-production editing? What techniques might a director employ to do this?

user3908: As you suggest, the scenes are shot in color. There isn't much of a special task for the director, it's almost completely a post-production challenge. In some cases a high contrast take can be made to assist in masking. This would be unusual for scenes involving actors, since the high-con pass wouldn't be well registered. It may also be possible in some scenes to shoot green screen (chroma key) to aid in separating some objects or actors.
The color portions are selectively revealed through masking, and possibly using some green screen and high-con elements. Using a combination of a full color layer and a mono layer derived from it, an artist paints masks that hide or reveal the appropriate layer(s) on a per-object basis. Computerized tracking and tracing can be a great help in this, but in the end it's up to the artist.

user2836: Jim Mack's answer hits on most of it, but you will also want to look at the term rotoscoping.  Generally speaking, this kind of thing is a highly manual effort to get best results.  If you can separate layers using green screen, then you can place distinct color elements in your black and white shot (or vice versa) but if they are all on the same shot, then someone generally has to go in and mask it out, frame by frame, by hand.  This is a time consuming, but not unheard of process.
Movies like Waking Life also make heavy use of rotoscoping.  The one nice thing about rotoscoping is that good rotoscoping software allows you to make use of the same principal of video that any modern video compression also makes use of.  That is, one frame is most likely going to be very similar to the next frame.  Thus, the rotoscoping software can start out with the mask being placed the same way as the previous frame and the artist only has to make adjustments to the mask to account for movement.  Some amount of automation may be possible for following strongly defined edges, but this often needs to be verified by an artist to make sure the edge tracking is good.

6512: <p>How is a movie made to show selective colors. For instance, in <a href="http://www.imdb.com/title/tt0120789/" rel="noreferrer">Pleasantville</a> a sister and brother are transported to the universe of an old black and white TV show, and the movie switches to being B&amp;W. Then, as the protagonists cause the inhabitants to have 'real' emotions, like fright, anger, or love, things (people, places, etc.) start having color. In the middle of the movie, many objects are still B&amp;W, yet many more are now colorized.</p>

<p><a href="http://www.imdb.com/title/tt0401792/" rel="noreferrer">Sin City</a> also used this effect (the whole film was initially shot in full color, and was converted to black-and-white. Colorization is used on certain subjects in a scene), for example.</p>

<p>How is this effect achieved? Is it post-production editing? What techniques might a director employ to do this?</p>
 3908: <p>As you suggest, the scenes are shot in color. There isn't much of a special task for the director, it's almost completely a post-production challenge. In some cases a high contrast take can be made to assist in masking. This would be unusual for scenes involving actors, since the high-con pass wouldn't be well registered. It may also be possible in some scenes to shoot green screen (chroma key) to aid in separating some objects or actors.</p>

<p>The color portions are selectively revealed through masking, and possibly using some green screen and high-con elements. Using a combination of a full color layer and a mono layer derived from it, an artist paints masks that hide or reveal the appropriate layer(s) on a per-object basis. Computerized tracking and tracing can be a great help in this, but in the end it's up to the artist.</p>
 2836: <p>Jim Mack's answer hits on most of it, but you will also want to look at the term rotoscoping.  Generally speaking, this kind of thing is a highly manual effort to get best results.  If you can separate layers using green screen, then you can place distinct color elements in your black and white shot (or vice versa) but if they are all on the same shot, then someone generally has to go in and mask it out, frame by frame, by hand.  This is a time consuming, but not unheard of process.</p>

<p>Movies like Waking Life also make heavy use of rotoscoping.  The one nice thing about rotoscoping is that good rotoscoping software allows you to make use of the same principal of video that any modern video compression also makes use of.  That is, one frame is most likely going to be very similar to the next frame.  Thus, the rotoscoping software can start out with the mask being placed the same way as the previous frame and the artist only has to make adjustments to the mask to account for movement.  Some amount of automation may be possible for following strongly defined edges, but this often needs to be verified by an artist to make sure the edge tracking is good.</p>
