[TheCaptain]

Happy New Year folks! Checking over the sheep today and I've noticed that one of the my Portland ewes has white developing on two out of four of her hooves on the hoof wall - I didn't want to get her in for a closer examination if it's nothing as she's in-lamb and don't want to upset her too much.

Ideas as to what it is???

[kanisha]

some colour genetics have an element of seasonal variability to them the proces of producing colour involves melatonin a hormone that is also produced in greater or lesser amounts dependant on daylight hours.  Sometimes I get golden retrievers whose nose changes colour from  black to light tan when moving from different parts of the world. I suspect this is all that you are seeing
The only thing I guess you might have to be careful of is that its not shelly hoof.

[princesspiggy]

my white gsds nose goes light in winter but our choc labs doesnt   

[kanisha]

to do with the type of pigmentation both golden retrievers, white sheep and white GSD's are  generally considered phaeomelanic ( light sensitive pigmentation ) Your chocolate lab is eumelanic different pigment different response.