There seems to me no greater challenge to a painter that brining together a clear balance of color dispersion and harmony.  What do I mean?  After one plans the color palette and executes the composition,  what remains is the ‘icing on the cake.’  How do you disperse color throughout the painting to give it visual unity?  I find this to be the greatest challenge and am still working on it.  Needless to say, as you paint dabbing a bit of what is left on your brush in correct and multiple value blocks livens the visual experience.   It is like pixels of every color you used dispersed throughout the painting.  Easier said than done.