Hi Lifeline,
We have plants on our roof terrace in Spain.
We chose mainly cactus and succulents.
They have survived for the last 3 summers, but, I must say that there have been times when we have been away for say 12 weeks, that they have looked a little sad.
But, given a big drenching, they have perked up again.
You could of course install an irigation system on a timer, but remember, water is precious on and costly in Spain!
There are some colourful cactus and succulents to be had, just watch out for the spikes!!!
regards,