Color Replacement Tool

Appears as the third option under the Heal and Patch tools in Photoshop CS's toolbar.

This is used to replace the color in the photograph with the foreground color selected and acts like a paintbrush to change the color. If you set the foreground color to black and the mode to hue it acts like a desaturation brush (below). You could previously do this by changing a photo from color to grayscale and then back to color, setting the History to the grayscale state and painting in with the History brush. Using the Color replacement tool is a much quicker way.

It goes one stage further by working like the Magic eraser where it only replaces the color of pixels within the same tolerance range as the first sampled point.

You can chose Hue, Saturation, Color or Luminosity modes.

Colorize

Turned on in the Hue/Saturation palette to create a monocolor image. If you want to tone a black & white image first convert it to RGB, Image Mode RGB Color. Adjust the Hue slider and watch the image go from blue through the colors of the rainbow and back to blue. Stop when the image is the color you like and save. Use lightness and saturation sliders to vary color further.

I've included a range of examples here using a black & white infrared photograph with the various settings made. You could use...