“To a timid student I would say, don’t be frightened of your paint — there is nothing so deadly as a thinly-painted tree. Put out plenty of colour on your palette and don’t compromise with the remains of your yellow ochre when another fresh colour is obviously needed.
“Don’t strive to accomplish everything in one painting. Sunlight and shade, colour, movement of leaves and texture of bark are all excellent things, but all are not necessarily incorporable at one attempt.”
Adrian Hill, “Drawing and Painting Trees”, page 128
I haven’t yet come across where Hill explains why yellow ochre will be the colour you’d have left on your palette, but you get the drift I’m sure.