You're right on both accounts Clint they are steel so can't be annoed and it's yellow zinc plating you're after, the silver one is clear zinc plating.
I'd just buy a matching set off the bay for a couple of quid, because I'll bet you BITD they just used what was at hand, yellow or silver.
Strange on this you're the first person I've come across who's after the yellow finish as most people prefer silver as the yellow mostly comes on lower quality fixings.