ff8822 is the one true YC orange.

The orange in the logo is ff6500, but it looks darker on the larger nav bar and with black text on, so you have to compensate.

ff9900 for me. (My first job was at a startup, that was one of our logo colors.)

try F6F6EF - makes the ycombinator logo stand out more

Or ffbb88

just for jokes! :P