Nemesis (Miss Marple) - Agatha Christie

Ok, I don't know where to even start with the list of things that annoy me about this book, but here is a question:


This is about colour blindness. It is my understanding that a person who is colour blind, specifically red-green colour blind can't distinguish between red and green and perceives both colours as roughly the same shade. 


It is my understanding that the condition would not cause a person to see red as green or green as red. 


Can anyone confirm?


Because amongst the many, many things irking me about the book, the following kinda had me puzzled.


‘No, you’re a bit colour blind, aren’t you,’ said Joanna. ‘I noticed that the other day.’

‘What do you mean, you noticed it.’

‘My red scarf. I asked if you’d seen it. You said you’d seen a green one somewhere and you brought me the red one. I’d left it in the dining-room. But you didn’t really know it was red.’