“…I had ranged the reds from pink to orange, which rose into the yellows as far as lemon with light and dark greens.”
— Vincent van Gogh writing to Theo about painting a portrait of Roulin’s wife, 22 January 1889
What I take from this is to not think of a colour as limited to only tubes marked that colour, but to include analogous colours too (colours that sit alongside one another on the colour wheel). So, not thinking of red as reds alone, but including yellow and orange as well as purple and blue. For blue to include green and yellow. Yellow to include green and orange, and so on.
Or put another way, the ‘secret’ to Van Gogh’s beautiful reds is to use more than only red.