Susan, in her blog entry http://plays-with-needles.blogspot.com/2008/06/zoo-inspiration.html
mentions a tool that SharonB originally mentions -
The Color Palette Generator.

Since I am thinking about sitting down with a DMC chart and lots of coloured pictures of embroidery, my ears and eyes rotated in interest.

Here's what I got when I fed in a slip that I had on hand -

http://jrm.cc/color-palette-generator/?image=images%2F1214178423.jpg&steps=5&method=precise

And with this flower, (imagine I wanted to do it in satin stitch using these colours)

http://jrm.cc/color-palette-generator/?image=images%2F1214180821.jpg&steps=5&method=precise

(5x5 box) it doesn't give me enough colours (there are obviously reds missing)

It's getting the reds in the 7x7 squares,
http://jrm.cc/color-palette-generator/?image=images%2F1214180821.jpg&steps=7&method=precise
but far too many other colours.

I'm finding the 5x5 configuration to be giving me the number of colours that I want in the examples I've tried.

So this tool may be useful. It at least shows a good sized block of each colour, instead following a line of cross stitch squares of the colour and I can be selective about the ones I pick.

I may use this in combination with 'hand' picking of colours - I'll see what works best.

http://wedgetalon.com/projects/drhacs/
converts the Hex numbers to DMC thread numbers.

Thankyou Susan and SharonB!