I used the following oil paints for washes:
I either applied them like a filter, for example orange on yellow to darken the yellow a bit and add to the transition to pure yellow, or as a wash to shade crevices.
Also, I applied both brown and green washes to green areas, for example, using the green to "just" add some shadows, using the brown shaded them and gave them another color layering slightly changing the appearance of the green.
All in all there was no fixed recipe, I looked at one plant and then decided on the spot which colors to use; sometimes mixing them on the palette first, with other models the mixing happend on the model itself.
Here are some shots of the final results:
This only leaves the bases to be done now.