What should be done to make a colour more neutral without turning it fully grey?

Correct answer: Add a very small amount of complementary colour

The governing concept is partial neutralisation through complementary colours. Adding a small quantity of the complementary hue lowers the original colour’s intensity while retaining some of its identity. For example, a little green can soften red without eliminating its warm character. A large amount of black darkens it, white changes its value, and equal quantities of all primaries may produce a dull grey or brown rather than a controlled near-neutral.