Just to add to Moonboot's nice answer.
ColorRamp can actually create the whole HSV spectrum for you :).

1. Set the two border colors (Red & Blue)
2. Set ColorRamp node to HSV > Far
3. Done

Just to add to Moonboot's nice answer.
ColorRamp can actually create the whole HSV spectrum for you :).

1. Set both sliders to Red
2. Set ColorRamp to HSV > Far
3. Done