I would say, There is a blue sign on the shop. When you see the shop, turn right.

'Turn right when you see a shop with a blue signboard ( Name of the shop, etc . 'Turn right when you see a shop with a blue advertisement sign ( inside /outside or on its wall/glass panel)

Yes, this sentence is correct. I would change only one thing: "Turn right when you see the shop with a blue advertisement."

green?

广告牌，billboard turn right when you see a shop with a green billboard