I frequently use empty water bottles to carry juice, iced tea, etc., and fairly often have such a bottle in my briefcase on the 6 train. I don't take it out and drink it on the brief subway ride), but I very often do on inter-city commuter trains which I do a round trip on once a week. There's nothing the least bit odd about this, and for people who travel longer distances on the subways, and in some cases have spent the day at manual labor in sweltering heat, it would be perfectly normal to drink on the subway from a bottle you refilled with a colored beverage.
I know..Ice tea would be a good example.....I was referring to the timing.