"Getting an Aircraft’s Attention in the Wilderness" recommends basic blue and gives photographic evidence:

http://www.fortunebay.org/getting-an-aircrafts-attention-in-the-wilderness/

While "Can Anybody See Me - Signalling for Dive Operations" recommends yellow for on the water:

http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/7727/SPUMS_V31N2_13.pdf

I personally favor fluorescent pink for terrestrial applications, such as flagging a tree, in most environments.

The common denominator in all of the above is contrast.