I live in a rental property that has a tiny yard out back. I’d love to paint it just to brighten it up a little more for sitting out in the 3 days of UK sun, but don’t want the landlord to take a hissy fit and steal my deposit. So if I did paint it, is there a (easy) way to strip it before I leave? 
No. Once it's painted, it's painted (otherwise any sort of harsh weather would strip the new paint).
The only thing you can really do is to repaint in that lovely gray before you leave.
Thought that was going to be the answer ☹️
It’s just the gross sloppy cement colour at the minute. Maybe I’ll give them a call and see if they’d be ok with it haha
The other thing you could do is a temporary something that you can remove when you leave, like a trellis with plants or a weatherproof print of something.