I saw some hydrangeas in western Cornwall that were turquoise and a few pale
green ones, could that be due to the copper / tin in the area? I have never
seen them those colours before.

Earnest

I've seen these coloured varieties in garden centres and it seems that that's
just the colour they are. They may change their colouring somewhat though if
they have the wrong conditions, but it's not something I have personal
experience of.

Dave.