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...

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

Sophisticated start. You could go with black and white photography, architectural pieces, art pieces- anything you love on the wall. Then, add pillows in the colors of the art work. If you want to stay more monochromatic, think in terms of textural interest, such as a shag area rug, pillows with seam detailing, etc.