Thanks guys. Especially Dave for that link to the colours. I looked at the Mapeil web site before I posted, knowing that people would shout Mapei - but I've got to say they have the WORST website I've ever seen (ok, exageration - but it is certainly one of the worst) it gives no indication of any colour chart, or at least I couldn't find one. It was hard enough to simply find the sealants.
In fact, when I got to my job this morning, the customer had already found some dark chocolate sealant - that we ended up using. He got it from Homebase. Their own product. Gotta say it was a good colour actually. No doubt its probably made by Mapei or someone similar anyway.
Did the job nicely. Thanks again. I've downloaded the PDF version of the Mapei guide for future reference.
Also found a company called SolSeal. May be of interest to some.
seem to have some interesting colours (or at least that's from squinting at the tiny colour chart) I've ordered a full size one.