Yeah, I had trouble choosing enough different colours (it's obvious I'm not a designer!), with the result that it's hard to distinguish between them

I decided to stick to the "modules" used by the schematics PDF, rather than try to invent/decide my own component grouping.
I guess the alternative to one-image-with-lots-of-colours is to split it into lots of separate images, with just one module highlighted in each image, but my gut feeling is that 19 separate images will just be too unwieldy.
I'll have a play to see if I can come up with anything clever in javascript

or if anyone wants to try doing something "clearer" with this image themselves, the source GIMP file is on
http://elinux.org/RPi_Hardware......2F_Layout - you'll want to play with the "Select by Colour" tool on the "Modules front" and "Modules back flip" layers.