Kérastase Micro-Voile Protecteur
But, it is a bit of a nightmare for the coloured hair. My hair seems to leach colour with one glance at the sun. But a year or so ago my hairdresser recommended this product and it is excellent. It's not necessarily something you'd use everyday (unless you were on holiday and sitting in the sun all day) but it's great for when you do plan to spend a lot of time outdoors.
You spray it in and leave it. It acts like an SPF for your hair. And it does make a difference. This is what the website says about it:
"Kérastase Micro-Voile Protecteur is an anti-photo degradation sun care treatment for coloured hair with low sensitivity. This transparent, non greasy spray protects hair exposed to the sun, sea and chlorinated water, leaving it soft and supple."
Coloured hair with low sensitivity? Crazy talk. However, it works. You can buy it here.