Tradition from approx mid 1800's to paint troop ships with a white hull and coloured band. Each ship had a different colour band. However, the blue band became the standard definition of a troop ship+ white hull + yellow funnel. Original troop ships were built for that purpose only by the Govt. but in later years were chartered as an when needed to save cost.

Interesting - was it the stripe that determined it to be a troop carrier? And why would that even be allowed during that tumultuous time?