Like James said, it depends where you are.
In Australia, Australiam Standard 1345:1995 (available on-line from www.standards.com.au) would generally permit the choice of (a) paint red all over, (b) paint red bands located as for markers or (c) pipe markers (white letters on red background for...
If it's for dust explosion, recommended reading is Lunn, G., "Dust Explosion Prevention and Protection - Part 1 - Venting", 2nd ed., Institution of Chemical Engineers, UK.