Whittock, Nathaniel: The Art Of Drawing And Colouring From Nature, Flowers, Fruit, And Shells: To Which Is Added, Correct Directions For Preparing The Most Brilliant Colours For Painting On Velvet, With The Mode Of Using Them, Also The New Method Of Oriental Tinting ; With Plain And Coloured Drawings (London, 1829)

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when the drawing is in this state the fibres of the leaves, &c. may be
marked with this tint, and the drawing is in a fit state to colour.
The colours required to tint this subject are prussian blue, gamboge,
yellow ochre, and crimson lake : a little of each colour should be
rubbed in separate saucers, and the tints formed by mixing them on
the palette as they are required. The bright tint for the leaves is
prussian blue and gamboge, mixed to form a fine bright green; with
this so over the front of the leaves. The backs of the leaves are tinted
with a light green, but very little removed from a grey ; this may be
formed by mixing a light tint of blue and yellow ochre. A thin wash
of this tint will be sufficient, as the neutral tints previously laid on
will give it strength. The ssowers are coloured with lake alone, and
when dry re-touched in the dark parts with the colour, rubbed up as
strong as it can be produced without being quite opaque; this will
glaze or lie over the dark neutral tint, and give great force to the
ssower.
The student will perceive from this lesson that it is not the gaudy
colouring which is the most essential part of flower painting, but the
drawing and putting the subject in light and shade. When this is
accomplished, the colouring is comparatively easy.