Great tit with no yellow, and day-glo greenfinches?!

Hello,

I'm new here, and don't have any pictures to post just yet but hoping someone can help.

I have a bird table with many visitors including blue tits, great tits and long-tailed tits. Today I've seen what seems to be a great tit but has a white chest and grey back (i.e. not yellow chested with green/blue back). Is this just a variation in colouring?

We also have greenfinches that look fluorescent yellow in flight. My husband insists they're 'wild canaries'.

Is there something in the water around here??