The easiet way would be to convert to RGB and use a third party plugin like Akvis Coloriage. But if you want to use photoshop you can magnify the image and use your selection tools to draw a lasso around the flower. Then set the foreground color to the color you would like the flower. Click Edit-Fill. Under "contents" use foreground color. Under "blending" the mode is color and the opacity is up to you. For more control over coloration another way would be to draw a lasso around the flower and use your brushes in color mode to paint on the exact variations of colors you want along with varying opacities.
