Sketch your rose and using masking fluid (sparingly) outline the shapes and small areas that should remain white (tone 1)

pink rose photo
pink rose1a   pink rose2
Allow the masking fluid to dry. Paint the whole rose shape with a very dilute pink. This should really only be coloured water, we don't want a huge difference between the white under the mask. (tone 1+)   Paint each petal in turn allowing different tones on the petal. Maybe add some very pale blue or yellow in places. Wait until 1 petal has dried before painting adjacent petals otherwise the paint will flow around
pinkrose final   When you are happy that the tones match the photo put in the foliage. Then put your painting somewhere where you can look at it for a couple of days while you decide whether it needs more leaves or not. If in doubt paint then in pale tones to start with.