With watercolour painting leaving white paper to indicate the lightest areas gives a real sparkle to the image.
In these calla lilly images all the white in the image is dry white paper.
When painting white objects in oil, I like to use burnt umber, yellow ochre or burnt sienna in the underpainting, which even if completely covered, will give substance and warmth to the finished image.
In this large oil on canvas, part of a series I painted a long time ago, the foundation block for the painting, was a monochromatic underpainting using burnt umber, yellow ochre and white. Without the underpainting the finished image would lack substance.

Working from a 'ground' whether in watercolour or oil also gives depth, warmth and substance when painting white objects.
In the following very quick sketch of a white lilly, I am working on a cadmium orange 'ground'. When I work this way with water colours, I 'ground' paper ahead of time so that it is completely dry when I begin to paint the image. To achieve white, I use designer gouache permanent white.

It looks quite grey and overcast today with showers forcast, and so a lovely day to paint!
A Bientot
catsInVienna
Oh I love your lilies! The last painting is also great, this tiny details on the cup! Lovely.
Enjoy your day!