Monochrome great tits?

Hello everyone :)

I've noticed the great tits in my garden have been very uncolourful recently, more white and grey than yellow and green. Otherwise they look just like great tits, larger than blue tits/coal tits and with the broad stripe down the front. I can't pinpoint exactly when this change happened, but I'm fairly sure they were more brightly coloured in January/February than they are now.

I read on another thread here that the yellow colour is caused by carotenoids in their food, and that this is less present in bird seed mixes. Is this an indication that they've had to rely on bird seed a lot over the past few months? Maybe I'm leaping a bit here.

Can anyone with more knowledge offer an explanation?