Windows: RAL 7002 Olive Grey — a soft, earthy grey-green that works beautifully with the render and slate.
Front door: RAL 6003 Olive Green — slightly deeper and richer, giving the entrance some definition.

Do you have lots of colour inside your house? Or is more neutrals? Choose something for the exterior that works with the vibe inside the house. If you painted the render as well it could look really sweet. If you just want to smarten it up without making a huge statement in the street, then paint them white - not PBWhite, but something like Wimborne White or Schoolhouse White or one of the murky F&B whites - see which ones work best with the render colour.

Here's some insp pics though - i googled for blue windows, with/without white walls...but you could try any colour that you like really...