Just my guess...
Maybe the Orange boarding pass is NOT an SQ boarding pass but a SATS (Singapore Air Terminal Services) boarding pass?
I remember reading the "procurement" section of SQ's website and they were looking for tenders to print their SATS boarding passes. SATS boarding passes may be used by SATS to check-in 'other airlines' that they do the ground handling for but do not have the boarding pass stock for.
Or maybe the passengers were only "connecting" in SIN and these "orange" boarding passes were issued by another airline in their originating city? (perhaps Jet Airways in India? I think they have through check-in agreement with SQ)