Painting these flowers in a friends garden was a fine way to spend a bank holiday Monday! 
When I began this painting I had no idea whether it would work as I wasn’t sure how to tackle it. 

This is how I started, by blocking the two main greens. The tricky part was the tonal values, purple is a naturally dark colour, so you lighten it but it takes away the colour saturation its a fine line to make it all work together.
Here is my tip:
If you are struggling with tonal values in your painting take a photo on your phone of your subject and your painting. Change from colour to black & white this way you can compare the tones to your subject and you painting and see if all the areas are working, and whether it needs to be adjusted, this is early stages:

The colours where ok but the tonal values of the colours too similar.

So I darkened the background and lightened the petals and Iris foliage area.
I also warmed the flower colour to pinky purple which goes better with the green.