The easiest way to paint a rose is to use a mask, especially in the rose itself. This way you can get clean crisp edges on each pedal. Work from the inside of the rose out and pay attention to the tones of color between each pedal to give it depth. Just cutout each pedal and work each section as you go. Once you get them all mapped out then you can comeback and make any changes freehand to soften any edges if needed and make any tonal changes also.
Another tip is is youre painting on a white, and the rose is red, hit the sections with a coat of orange, then hit it with the red. This will stop the red from shifting to pink being on white and will give you a real rich red.
Hope this helps a bit!