Photoshop: black and white icons with colored parts using History brush

In addition to what has been suggested here, I tried to make a tutorial regarding the use of the History Brush; there a lot of ways of doing something in Photoshop, one more can only confuse you help you.
I used PS7, but it's the same for PS6.

the original (well, cropped and resized actually):


I'll show how you can:

I. grayscale certain parts of an image:

I made a copy of the original layer, just to make sure I still have it for comparison.

1. Desaturate the layer:
Image -> Adjustments -> Desaturate.

2. Set the source for the History brush, in the History window, to the Desaturate stage in the "history" of your image:

If the History window is not visible, select Window -> History (or Window -> Show History in PS 6).

3. Change the current history state using the sliding cursor as shown in the image:

or simply clicking on the desired state. It will probably be the previous state right before desaturating the image.

4. Use the History brush on the parts of the image that you want to be grayscale. (you probably know it, but this is the History brush:
)
It's a brush like any other in regards of shape, opacity, diameter etc.
For teaching purposes I grayed the guy :)
You may have to change the brush diameter for precision, but it shouldn't be too difficult, given that the icon area is relatively small.
result:



or
II. preserve color only on certain parts of the image:

all in all, it's pretty similar. but still:

1. Set the History brush source to the current state of the image (here it happened to be when I resized the image)


2. Desaturate the layer (Shift-Ctrl-U)

3. Use the History brush on the parts of the image for which you want to restore the original color.
I colored the blue crystals, using the brush at 19 respectively 13 px:


I hope it's useful. I welcome corrections and suggestions.