I agree with OP its difficult to get below 6 with a Colnago. But whats the point when you have a solid stable foundation of a Colnago, why mess it up with sub par components?
I was able to get my 61 traditional c59 to 6.8kg, but not without compromises. For instance I used LOOK Ti blade pedals, very light, but went back to trusty Dura-ace within 100 miles. I had carbon bars initially. I've swapped them to alloy because I prefer the style, shape and a little more stiffness, or so it seems. Saddles are a no compromise thing.
+1 the key is compromising durability and fit for the sake of gram chasing.
I am in the same boat as Colnago. I had my C59 down to 6.25 but its wasn't solid anymore. Some WW parts are trustworthy, some are not. Not
-Time Xpresso 12 pedals back to DA 9000 pedals +82 grams but they work every time forever
-MCFK bars to Deda Superleggera +22 grams but I can handle the shape of the drops
-Extralite stem to Time Stem +43 grams but I can handle wicked descents with cross winds and 40mm wheels
-I-link mini cables - just use Di2 and be happy with every shift forever. Trustworthy
Great places to save hundreds of grams of weight on a Colnago:
Expander plug -> MCFK or extralite saves 37 grams off stock.
Seatpost -> MCFK no compromise -100 grams off stock
Seatpost clamp -> MCFK -17 grams off stock.
Brakes -> EE brakes no compromise on steep long descents (with DA lever, some compromise with Campagnolo levers) -112 grams saved
Cranks -> THM, lightning or Super Record Ti can save a lot of grams with no compromise
Wheels -> any descent carbon tubular will save a lot weight over alloy clinchers. Pimp them out with Ti freehubs, SRAM Red cassette, no decals, superlight hubs etc.