Velocity of which of the following colours of VIBGYOR is maximum?

Red

The velocity of light in a medium is maximum for that colour for which refractive index is minimum. 

Clearly, the velocity of voilet colour of light is minimum in a medium and refractive index of that medium is maximum for violet colour. 

Similarly, the velocity of light in a medium is maximum for red colour and refractive index of that medium is minimum for red colour.