Someone has shared a very nice color picker utility called Colorette, which was written in AutoHotKey. The app description is at http://www.autohotkey.com/forum/topic69559.html and the EXE itself is http://www.autohotkey.net/~sumon/apps/colorette/Colorette.exe .
For an example of how to use Colorette with Revit, save the EXE to your Desktop and run it. Find a web page with the color you want to find, like http://www.jameshardie.com/homeowner/colorplus-palette.shtml , hover your mouse over the color on the screen and press Space on your keyboard. A color dialog will appear with the RGB values.
In Revit, find a similar material to what you want and copy it or edit an existing material. In the appearance settings, edit the Color of the material.
Type in the RGB (Red, Green, Blue) values that Colorette has found.
Check "Use Render Appearance for Shading" in the Graphics tab.
When you look at your material, it will have the color that you had selected.
Make Siding to Match the Manufacturer's Published Colors
If you want to change a "wood" siding material to that of your picked color, duplicate the "Siding - Clapboard" material (which is in the Residential template) with your new name.
Change the material's image to "Thermal_Moisture.Roofing_Siding Panels.Wood.Horizontal.White.png".
Move the "Image Fade" down to around 15.
Get your color with Colorette.
Edit the color's RGB values. The color will mix in with the siding image to make the new material appearance.
A rendering of the result: