How to colorize from black and white.  This questions comes from a Photoshop Designer in India.

Good question pravin;  First, make sure you are in the RGB mode so you can add color.  Then you'll have to select areas that you want to colorize using any of the selection tools (remember you can use "Add to selection" setting in the options bar to create many little selections or areas of the image to apply the same color to).

 

You can go to Image: Adjustments: Hue/Saturation (shortcut Ctrl or Cmd U) and check the colorize box.  Now move the slider towards a desired color.  You can also desaturate (lose color) or saturate (make the color more intense).

 

There's no easy answer because colorizing is going to take some time and handiwork and depends on your situation.  You're going to have to get in there and make selections by hand to bring in different colors (kind of like animation, very painstaking).

 

Definitely try out some blending modes and lowering opacity as this might help the color work better with the layer beneath (when using layers just of color or adjustment layers).  I would use hue adjustments layers (click on the icon or use the menu after you have made a selection), otherwise if you colorize a layer itself make sure you have a backup copy of the original layer and create a new layer just to paint on.

 

In addition to 'selecting' areas and using the hue "colorize" function, you can also really make it hard on yourself and use the paint bucket (to fill areas) or different paint brushes (this works best for vector graphics). 

 

To make it easier, have a document open with the similar colors that you might want to reproduce.  This way you can just keep choosing colors by referencing the open color document with your eyedropper tool (shortcut "I").  Then you can use your paintbrush to paint in areas.

 

It is so much easier to colorize vector graphics with simple shapes/objects (for ie. cartoons) and slightly harder with photos that have simple, distinctive elements (a big sky and water).  

 

Now, if you have a "busy" photograph it's going to get harder to take the time to colorize, but how much effort you want to put into it is up to you.  It's going to take some time.  You can use the magic wand tool to select larger, defined areas to colorize. 

 

Each different area you select you would choose a different colorized "hue" adjustment.  I'd recommend a separate adjustment layer for each color and then put them all in a layer set for organization, this way you can make corrections easier.

 

If you want things to look more realistic when colorizing from grayscale to RGB, colorizing with a hue adjustment is going to look more realistic then "painting" a solid color onto, say someone's skin because it will map the color to the texture and properties of the selection.  It depends on your situation but remember you have blending modes and opacity to help.

Another way to colorize is to make a selection, create a new layer and then fill that selection with a color (or gradient) from the palette and change the blending modes.  Once again though, if you need different colors, it's just going to be a long process.  You could also try a gradient or color fill adjustment layer and change the blending modes but this doesn't give you as much control.

 

That should give you some advice in the right direction but a good colorizing job is usually going to take some time unless you want the effect of a "colorized" look (then it won't be that bad for you); in the case where you might want to just colorize a few elements and leave the rest in black and white. 

I always photograph in color now (when I do use my 35mm Canon) because I can easily use Photoshop to get the perfect black and white shot off of a color (how's that for you Photographers!) but to shoot 35mm in black and white and convert to color--yikes (stock up on No-Doz).

The new replace color tool in Photoshop CS should work well; just get your source color from an open document or from the palettes and it will paint in fairly well to match the contrast and light quality of the black and white image.