I have to agree with Doug regarding rib tape colour. Think about it. Two layers of CDL cloth are going to be more opaque than one layer. More opacity means the rib tape will stand out and appear lighter in colour against the surrounding area.
Take a look at the picture below. SE5a from RAF Hendon. The picture shows the lighter rib tape clearly. This effect is more pronounced when you are staring at the real thing.
Cheers,
Bob