If you are following Bryce Mohan's posts on Layer Masks in Photoshop, you know the man is a master artist.  And if you using full version Photoshop, the Channel Mixer is right here:  Layer -> New Adjustment Layer -> Channel Mixer:

But, what if you are using Photoshop Elements?  Well, you had to know there was some reason Elements costs about $60.00, while the full version Phhotoshop CS3 costs about $600.00.  The reason is Adobe stripped a few things out of Elements to make it affordable for the average guy or gal.  Channel Mixer was one of the casualties.

But not to worry.  We can kinda, sorta, almost get you to the same place in Elements without the Channel Mixer.  Without even installing Hidden Elements set of secret power tools.

Try this:

1.  Open your image.  Choose one that is colorful to start with, so you can better see the effect.

2.  Hit Layer -> New Adjustment Layer -> Hue/Saturation

3.  Either go with the default layer name, or choose a new name and click OK

4.  The Hue/Saturation dialogue box will pop up.  Drag the Saturation slider all the way to the left side (-99).  Click OK.

4.  Notice you now have a new adjustment layer in your Layers Palette. .

5.  Click the Gradient tool.

6.  Notice the Tool Options bar.  Click on the drop-down arrow, and choose the Black,White Gradient. (You will want to come back later and experiment with the other gradients, too).  At "Mode:" I set the Opacity at 50%, also be sure transparency is checked. 

7.  Click at one edge of the image, then drag the gradient line across the image.  You can drag all the way to the other edge, or you can stop somewhere before the edge.  I stopped about 1/3 of the way across.  How far you drag the gradient, whether you drag at a straight line or diagonal will all give you different results.  You'll want to experiment to find what works for your particular image.

8.  When you stop dragging and release the mouse button, the image will change, revealing partial color, fading to black and white.

You don't have the kind of precision or control you would have in the full version of Photoshop, but whaddaya want for $60 bucks?  <s>