In the end I chose the Mirasol Sulka yarn in Thyme (212), and I wanted a pattern/stitch that was both simple but quite effective visually. I tried a couple of stitches and ended with the herringbone stitch. This tutorial is really useful. Once you know it is easy but requires a lot of work for your elbows though. Beware this stitch eats yarn, but the effect is really worth it.
My big issue was to make the cushion reversible. Once I decided that there was too much yarn going into this I decided to go for a geometric pattern at the back. Here is the back (already quilted) on its own. It is my first attempt at this design, so it is not my best effort but you have a good idea of the effect.
And here is the corner of the cushion! it is actually not as bad as other parts of the patchwork. I love the butterflies fabric (I think it is a Michael Miller fabric). The colours are a bit richer than shown on the picture.
I will definitely used that pattern again, I am hoping to make a big wall hanging patchwork for the new house, with a mix of light and dark colours of the same tone.