Ooh, another Darwinite!!! Which route are you doing? I did the VDLP and continued on to Finisterre. My favourite top was a dark blue Montane Bionic which contains merino. It was excellent even during the worst of last year's heatwave (I went in June-July). I had an Adidas thin running top which I thought would be better for hot weather. Great top but having pack straps rub against it damages it quite a bit and I knew from about halfway which top I was going to burn at Finisterre. You will be handwashing most of the time and drying on lines so pick a colour that doesn't show the dirt as badly. White is not a great choice
. The merino tops are also less smelly.
I did the Rogaine event last Saturday wearing my favourite Camino bra to realise that Spain was obviously a lot drier than Darwin as I got a bit of chafing because the moisture couldn't fully wick away. I was sweating from the moment I stepped out of the shade. That never happened in Spain even on the hottest days.