As the appearance of the garden has a modernist style with the straight lines and concrete furniture how about a Mondrian influence and add a bold strip of blue? It is a pond and with the right shade it could look really relaxed and contemporary. My daughter's suggestion is a chalky lilac!

Deep gray and all the other accents in silvery old wood and cream. This will add drama, allow the other elements to bring ion light and set off the concrete wall as a true feature. If you want light instead of drama, then go for a very pale beige stone. It will echo stone colours in both the concrete and rocks...

I would go with a gold or yellow ... of course I happen to love teal/ turquoise and gold/ yellow together. Something in variations on the photos below... maybe some pink accents if you have a girl or lime green or blue accents if you have a boy.

Small all in one condo room not sure what color to use for walls after removing wallpaper