I have to go along with Chris, I think it is most likely the breakdown of the chlorophyll. Although the broccoli flower is yellow, they grow on the opening florets like this:
I don't think this is what Chris was referring to in the 'yellowing'.
BTW, BC, if you were married to my Mrs, you'd be in a living hell. I'll swear she's got shares in a broccoli farm! Broccoli with this, broccoli with that, broccoli with broccoli, broccoli with just about everything and anything. If they made broccoli ice cream she'd buy it.