First, get a black & white picture. I'll use this one:
Grab the Freehand Selection - tool from the Toolbar.
Then choose the area you want to colour with the tool, like this:
Then go to Adjust > Hue And Saturation > Coulourise ( or just press Shift + L ). Colour the area you chose to the coulor you desire it to be. Then it should look like this:
After this, coulour the whole picture by repeating the last step ^.
I came up with this as a final result: