Switch to your “Channels” palette.
Hold down Ctrl and click on one of the channels. You will see that you have now selected the white areas of your picture.
Invert the selection, Shft+Ctrl+I. Now all the line art is selected.
Go back to your “Layers” palette and create a new layer (Photoshop Create New Layer).
Get your Paint Bucket Tool and fill the selection black. Now you have black line art on a transparent backing.
Deselect, Ctrl+D.
Create a new Photoshop New Fill Layer at the bottom of your “Layer” window and select Solid Colour…
Select a colour in the Colour Picker dialogue box. Click OK.
The new fill layer created will appear in your layer window.
Fill the layer in black using the paint bucket tool.
The color will now disappear from view, use the polygonal lasso tool and pin it around the section of the image you wish to color.
Proceed to select your foreground color back to white which can be done by using the shortcut keys 'D' for a black foreground and white background color and 'X' for a white foreground and black background color.
After you have selected white as your foreground color use the bucket tool to fill the pinned section via polygonal lasso tool to color the chosen section of your image.
You can also use a hard brush to color your image but this is a lot more easy way, and if you leave out some parts you can always use the brush tool to cover it up.