There's a thread on this recently:
http://www.theaerodrome.com/forum/sh...ad.php?t=17874
If you browse the thread you'll see some links to some tables that I scanned in from the back of a Methuen book which cross-references the Methuen codes to Munsell codes - these are supposedly precise conversions.
You can then download a free program, also linked in the thread, which allows you to convert the Munsell codes into something more usable on a computer, such as L*a*b or RGB. It's a bit convoluted, but you'll get there in the end.
Of course that'll only be approximate unless your system is properly calibrated and some colours won't be representable on a computer screen either, but given the nature of the subject most colour references are likely to be at least a little approximate in the first place.
Have fun
Finn