I read on the Adobe forum that there are many people who make a big fuzz about making duotone images for the web. (a duotone can for example looks like an old sepia photograph).
Me, I start with my document in RGB. I convert it to monochrome with the Channel Mixer (using the sliders to get what I like), and then I change the image to LAB mode.
In LAB mode, I go to the Channels Palette and select first the two colour channels, and then finally the brightness channel with levels (Ctrl+L).
Then back to RGB and save.
Must say that I use LAB more and more.
I post herewith one of my Labyrinth drawings, set to some sepia colour.
As an extra, I give you the JPG File Sizes:
100% in Save for Web, PS7: 190KB
100% in ProJPEG : 176KB
85% in Save for Web : 119KB
85% in ProJPEG : 62KB (the one I posted)
duotone 
Linear Mode
