Very, very nice update: going through your 
gallery is like an afternoon in the Prado for a 
CLAMP freak like me. 

But your "Dark Card" cel raises my curiosity.  
Yes, it looks "dark," but in fact it's The 
Light Card.  You can tell because Light 
has the crown with the big point in the middle 
and Dark has the gap in the front that makes the 
points on her crown look like antlers.  And 
actually both Cards have white crowns and white 
collars.  Snow Bunny 
 has a cel of 
Light in her "Clow Card" section that has the 
same odd characteristics as yours.

So what is this?  I'm going to have to look 
through Eps. 42 and 69 (the only two places where 
this card appears) frame by frame, because I 
suspect that there's a place where the animators 
create an effect by rapidly intercutting a 
normal "Light" with a partially negative-
tinted "Light."  Sort of like the 
infamous "flashing Pikachu" moment that allegedly 
caused thousands of Japanese kids to have 
epileptic fits. 

Your cel doesn't give me the fits (though your 
Wish cels make me want to rob a 7/11 and head for 
an anime convention). But it does make me very, 
very curious.