A shadow colour became lifeless after adding only black. What should be done for richer chromatic depth?

Correct answer: Mix a suitable cool or complementary dark colour in a limited amount

The governing concept is chromatic shadow modelling. Black can lower value, but using it alone may reduce saturation and make a shadow look flat or lifeless. A small amount of a suitable cool or complementary dark colour can deepen the value while preserving a relationship with the local hue and surrounding colours. Adding more black repeats the problem; white lightens the mixture, and erasing the shadow removes essential form information.