Transition metals are well known for having more than one stable oxidation state.
Vanadium, like many transition metals, has several different oxidation state and, like several, these oxidation states are of different colours
Also like other transition metals, these oxidation states are coloured.
In this demonstration / experiment vanadium 5+ is reduced to give the 2+, 3+ and 3+ states and then sequentially oxidised back to the 5+.
