Tulips before the rain

I just managed to photograph these two tulips in my front garden before there was a sudden downpour... it had been bright sunshine two minutes earlier and it is again now. I know my memory isn't as sharp as it used to be, but I'm sure this tulip was all yellow last year - in fact, I know it was! It was a rogue yellow tulip in a pack of deep red ones that I bought and planted several years ago. (You can see the original colour HERE.) So why has it now decided to introduce red into its colouring?