This could be an interesting tutorial.
Using this you can color almost any Black & White image.
Actually there are two ways to color an image. You have to use them according to your needs.
The methods are sorted with simplicity.

I method: Simplest

Adobe Photoshop has a great blending mode called the color mode.
It can give fantastic effects!

Step 1: Select the color you want to do to a specific part of image
Step 2: Select "Brush Tool" {Size: Choose according to your needs --- Mode: "Overlay" or "Color" --- Opacity: 100 --- Flow: 100}
Step 3: Go nuts! Just paint the area of the picture you want to color

Pros: Easiest coloring possible, well for newbie’s who don't want the resultant image to be too realistic.
Cons: Very time consuming, provides pretty cheap results, you can't adjust the colors because you'll have to paint again...

II method: Hardest

Remember that in this method you'll have to work with objects that are in the front and then go to the objects in the background.
Here I'm taking a picture that comes with Adobe Photoshop as a sample.
You can find it in [Drive]:\Program Files\Adobe\Photoshop CS\Samples\RanchHouse.jpg

Step 1: Change the image to color mode by Image ---> Mode ---> RGB Color (!!!IMPORTANT!!! Always check this)
Step 2: Zoom to about 200%
Step 3: Select the magnetic lasso tool {Feather: 1px --- Anti-aliased: Yes --- Width: 10px --- Edge Contrast: 10% --- Frequency: 100}
Step 4: Now click on the edge of any object Coat, Boot, Hat, Door etc... (Not the wall or the floor)
Step 5: Just move your mouse cursor around the edges of the object until you have a complete selection. Like this
{Tip: If your magnetic lasso tool doesn't go where you want it to, you can just click the points manually. To remove the last point just press backspace. Refer to Photoshop manual for more assistance}

Step 6: Press Ctrl+J to do "Layer via Copy"
Step 7: Go to Image --> Adjustments --> Hue/Saturation {Colorize: Yes}

Step 8: Now adjust the Hue/Saturation and Lightness until your object looks realistic.
{Tip: The realism of the resulting image depends on the experimenting you have done. And the amount of efforts put to small objects. Always remember to experiment with the "Brightness/Contrast option to make your objects looks better}

Step 9: Set the blending mode of the layer to "Color". You can experiment with others too...

Step 10: Do the same for every other object. (Remember to click on the Background layer first)
{Tip: Using "Magnetic Lasso" tool won't always do the work ... It is difficult to select the images using this tool...
So after using this tool... select the "Rectangular Marquee" tool and select the "Add to selection button" (Hold your mouse over the buttons to make the text appear" and then draw at the edges}

Step 11: To color the background (Ground, Wall) you need not worry about the selection too much because you have already colored the objects and put them over the top of the background...
So Click on the Background layer and just select the wall including the objects and directly apply "Hue/Saturation" to it...

Step 12: Layer ---> Flatten Image
Step 13: Image ---> Adjustments ---> Auto Levels & Auto Contrast & Auto Color

This should make you able to color most of the B&W images to color.

Tutorial written by: Nikhil Verma