Hi Glenn,
The fabric was attached to the wings in one of two ways. Earlier aircraft often used wooden strips, which were tacked through the fabric and into the ribs. You will see these strips on both upper and lower surfaces and they stand proud. The later and most common method was for the fabric to be glued or stitched into place, then after the fabric had been shrunk, the rib areas were reinforced with another strip of fabric. This strip is the rib tape. The effect varies depending on what sort of finish was used on the upper surfaces. If the whole wing was treated in clear dope (CDL) then under bright sunlight the ribs on the underside would show as shadows, ie darker. If the upper surfaces were treated with a dark colour such as PC10, this prevented the light shining through, and the double layer of lighter linen over the rib area tended to show lighter underneath.
This can be seen clearly on these two pics of aircraft displayed next to each other in the Imperial War Museum, London and lit from above by sunlight shining through the glass roof of the atrium.
You'll see the same effect on pics of The Vintage Aviator's aircraft on their site.
Similarly, the fabric on the upper surfaces will appear slightly different in tone due to the double layer of fabric.
This is the same Camel shown above. You can see the slight difference, but it is nowhere near as obvious as the underside.
This link may be helpful too...
http://thevintageaviator.co.nz/reference/working-irish-linenHope this helps!
Ian