We have the same problem. It is recommended to spray with Epsom Salts to get it to darken as it should be.

Owd, do you think I could risk planting it out? Thanks for the suggestion.

As its in a pot I would plant it out with as little root disturbance as possible unless it is pot bound. they don't like being moved around ie dug up and replanted as it takes them quite a while to make the new fine water take up roots.

if you think about it when you bought it, it had to be transplanted into your ornamental pot.

Is it in a lot of shade? Perhaps moving it into a brighter area will help as will the Epsom salts. I have several in a lightly shaded border that only gets direct sun late afternoon in the height of summer.

Mature plants can be problematic to transplant, but these youngins should be fine. They look like they need it. If the ground can be worked, go ahead and transplant being careful not to disturb the rootball. Maybe go around the perimeter of the pot with a butter knife to loosen it, then the whole thing should plop out. If that doesn't work, consider cutting the pot off or smashing it with a hammer if you don't care to save the pots.

I will take all the advice on board. Thank you all.