So you don't need a whole spectrum of colour (my first mistake) in fact the more colours you use the harder your task in conveying form. But finding just the right hue is no easy task and takes a lot of trial and error. This weekend I have tried copying one of his pictures with pastel pencils on a ground of black acrylic. It's been a fun exercise to find which shades to use for flesh colours and I'm modestly pleased with the result.
And I can't help thinking that fiction is a lot like that too. Hit the right note with your characters, give enough light and shade, and your story will come to life as you'd never expect. Too much and nothing works.