You can dig them up now if you wish, whilst they are in the green.
If you have the choice, just wait a few weeks until the tops have died
down and all the energy from the leaves has transered back to the
bulbs, then lift. This would be my prefered choice.
Having said that, if buying snowdrops, they are invariably sold as in
the green, which is very acceptable.
Clifford
Bawtry, Doncaster, South Yorkshire