Have a look at the Aquamix website;
Aqua Mix - For Home Use - Products and have a read about the different sealers, and make sure you give your tiler plenty of warning as to what you expect him to do.
If you want to enhance (and darken) the colour of the travertine use Enrich n Seal (but there's no going back if you do!), but if you just want the stone to look as it's installed, then use Sealers Choice Gold. Both over premium protection, which means maximum reaction time to any spills.