In an essay at The Telegraph, photography critic and picture editor Lucy Davis muses about the palettes used by well-known artists.
Some artists mix every gradation of colour they will need for a painting before they start, others as they need them. “My freshly arranged palette, brilliant with contrasting colors, is enough to fire my enthusiasm,” noted Delacroix in his Journal in 1850. The French artist was meticulous in his arrangement of colours, and when unwell, would take his palette to bed and spend the entire day just mixing new shades.
The actual palettes of Renoir, Seurat, Degas, Delacroix (above), Moreau, Gauguin, and Van Gogh are illustrated, accompanied by commentary on how the physical layout of colors on the board may influence the figurative "palette" of color choices used by the artist for his work.
Link.
For example, if I want to do something in reds, yellows and oranges... well a healthy glob of each color is where I start and I blend as I go.