If at all possible, shoot under overcast skies or early in the morning before sun strikes plants. I have used both black and white umbrellas, and find that while they prevent the harsh lighting of direct sun, they tend to skew colors. Use proper white balance if your camera allows it. Use a telephoto lens (or the telephoto lens setting on your point and shoot). This will allow you to change the background of the bloom you are shooting, throwing it out of focus and making for a more dramatic image.
For many more details on white balance and color temperature, dynamic range and macro photography, check out the blogs on my two websites:
www.brooksgards.com
http://johnsphotogalleries.com
