This is a small demo (that turned into a diptych) of some hydrangea and roses in oil on canvas painted alla prima. The key is to block in the forms and values in the underpainting and not get hung up in details. This was done on a canvas toned with a light, grey green, which is complementary to the reds and pinks of the flowers.
After completing the under painting in burnt sienna, I began blocking in the darkest darks with cobalt and burnt umber. The greens are cobalt and yellow ochre, plus some cadmium barium yellow for the bright areas. I developed the shadow areas on the hydrangea first, using a yellow ochre, cobalt and alizarin with white combination. The bright petals of the hydrangea are white plus cadmium barium yellow. Be careful not to “dab” the paint on uniformly in a random pattern, instead look for the areas in light and carve them out with brush strokes rather than dabs. The approach for the roses is the same, working the shadows first followed by the lights.
I’ve kept these loose and abstract which is the way I prefer to paint flowers (and most everything else). I find working this way imparts strength and boldness, and keeps a subject that could get sweet from doing so.