The fastest way I know, is grayscaling the image, put a layer below it, and put the grayscaled image on multiply. Then add colors on the bottom layer...
If it's already coloured you can use the wand tool to select the colour you want to change then you can go to image>adjustments>hue/saturation make sure the colourization box is checked then adjust the sliders to get the colour you want.
Linear Mode
