Here is a short video to do  with mixing transparent complimentary colours to provide some versatile and exiting deep darks in a painting

In the above watercolour of Royal Pavilion, Brighton, set against a midwinter night sky, it was particularly important to give depth and some colour to the darkness of the sky. Had I used only a black, or a pre-mixed dark such as Paynes Grey, this would not have done anything to compliment the blues, mauves and yellows of the building.  In fact I used Burnt Sienna mixed with French Ultramarine, flowing into some Permanent Magenta and Indigo. This complex gave me enough deep, varied blackness.