What should be done if a colour is light but too dull and needs more vividness?

Correct answer: Add a pure amount of the same hue

The governing concept is saturation, not value. The colour is already light enough but lacks vividness, so adding a small amount of the same hue in a purer, more saturated form can increase chroma while preserving the intended colour family. Grey and complementary colour generally neutralize or mute a mixture, making it duller. Black would reduce value and make the colour darker, so it does not solve the stated problem.