I recall reading that after 9/11 the BBC had decided that the public reaction to the Queen Mother's death would not be as big, and so they wouldn't cover it as extensively or treat it as such, with Sissons' choice of tie colour reflecting that. I think the BBC misjudged it on this one, compared to Diana where they were very much in tune with the public's mood.
I've said this before but Roger Mosey's book explains why Peter Sissons didn't wear black. They removed the stipulation that they had to wear black because it was always assumed that the story would break gradually, first reporting she was ill, then finally reporting she was dead. They wouldn't wear black for the first bit, but they were supposed to for the second bit, immediately after the announcement was made.
However, it was decided that if that was the case, they would have to change in about thirty seconds, which was considered more or less impossible, and furthermore it was thought that them changing clothes halfway through would be distracting for the viewer, so when they made the announcement you'd be more distracted that they'd just changed clothes than paying attention to what they were saying. So it was decided that as long as it was suitably sober, it was alright to stay as they were, for a bit anyway.
Except when it actually happened, none of that build-up, which was the whole point of changing the dress code, happened, and the first they knew of it was that she was dead.