Apple blossoms are usually pink as buds and then start turning white as they flower; a few varieties are actually dark pink and some have just the edges that are pink and those are what I´ve chosen to paint. My photograph has the light coming directly overhead and the painting has it falling from the left, so I´ve changed the lighting accordingly for the flowers and leaves. As you are modelling each petal , imagine the direction of the light and ask yourself whether the area that you are painting will catch the light or fall in shadow.
For the first layer (protect the stamens with masking fluid), paint in all the form shadows first; I simply made up the folds and creases on the petals. Make a mix of the red and blue to get a violet and add yellow to dull the violet to a shadow color. Wet each petal separately, wait for a few seconds for the water to sink into the paper and lay in your shadow mix. After all these layers have completely dried, wet the petals again one by one and touch the edges of the petals with your red mix, as this starts to dry add the veins with a a slightly more concentrated red. The shape of the veins are very important as they suggest the form of the petal. The shadows that you have put in the first layer should show through in the next layer if you are using a transparent red; you can further define these shadows with a red-violet mix. I like this Cheap Joe´s Lizards Lick brush (in the photo above) a lot for painting fine details like veins etc, it has a sharp point and also has a good body of hairs at the top so you don´t have to load the brush so often. Remove the masking after everything´s thoroughly dried and paint fine shadows on the undersides of the stamens, paint the pollen with DS new gamboge and put tiny shadows in them:
The leaves are a mix of the two yellows and phthalo blue and a hint of the red: