In 1990, an assessment called Project Rainbow paved the way for HSBC (HSBA.L) to move from Hong Kong to Britain. As Europe’s biggest bank now considers moving back, the same exercise offers clues to its final decision, say industry sources and analysts. Project Rainbow assessed HSBC’s future base by considering whether it was operationally effective, tax efficient, politically acceptable, consistent with bank regulatory requirements, in the best interests of shareholders and compatible with any future merger of HSBC and the Midland Group.