Such a shame it is so, narrow, even an extra 10-15cm would have made a difference to your options. In addition to sorting out the drainage issue, I would also be tempted, before the project is finished, to turn the corner of the L into a square planter as no one will sit there as their legs would be squished. That way, you could at least plant one substantial plant, such as a small standard olive tree if you go for the Mediterranean theme. Extra expense, but I think I would be having words with the landscaper, if they came up with the width, to ask what on earth they thought you could plant in such a space and try to come to an agreement on additional costs.
You can keep it contemporary by using a single colour, but using plants with different leaf/flower forms. I would paint the fence behind a dark grey and use white flowered/silvery leaf plants (such as armeria achillea, fastigate or trailing rosemary, variagated ivy etc.) that would shine against a dark background. A dark background also recedes to the eye, a useful trick. Doesn’t have to be white of course, whatever your preference, but a riot of colours would look too busy and messy, I think.
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
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You can keep it contemporary by using a single colour, but using plants with different leaf/flower forms. I would paint the fence behind a dark grey and use white flowered/silvery leaf plants (such as armeria achillea, fastigate or trailing rosemary, variagated ivy etc.) that would shine against a dark background. A dark background also recedes to the eye, a useful trick. Doesn’t have to be white of course, whatever your preference, but a riot of colours would look too busy and messy, I think.