It would be far happier in the ground

It is really down to your personal taste. I prefer terracotta or stone., though a friend has a dark grey rectangular pot for an evergreen shrub.

As SBG says,its down to personal preference,and as MG says,it would be far happier in the ground.If you have to put it in a tub,I would try one of the large half barrel type,as it wouldn't transplant well,when it outgrew a smaller one.

Dark green can look good in stainless steel or copper which will change colour as it ages.

~ mine is in a large black plastic round one with ribbed sides after a repot last year and is doing fine~