Ok have gone through and found some photos to show how they change.
They winter out unclipped and unrugged as not in work, and natural coat colour is dark. Changes slightly according to how and when they were clipped that year, as type and timing of clip varies according to weather and workload. First pic here is from Nov 2013 and second is the start of Feb 2014, the longlining pics were taken Feb 2015. Winter-time Pif has very obvious white patches on shoulder and bum.
Springtime coat change. This photo was taken at the start of April 2014 - so this is what their spring coat looks like when they only had 1 clip the previous year. White hairs start coming in, mixing with the winter black and starting to make them a bit roany. Also a brownish tinge in places (Paf in particular gets quite a brown head in spring).
And so by May time almost completely white, just in time to start moulting it everywhere!
But here's some cool pics from this spring
In March we had a sudden, very un-Normandy-like heatwave and as the boys had just started driving again after their winter break and were unfit and hairy, they had a half-body clip to help with the sweating. So this is Pif after the clip on 14th March - back end still in its dark winter hair, front end newly-clipped, and between the two you can see the white spring hairs coming through. If you look at his belly you can also see how thick his winter coat was - this is why they need clipping when they come back into work!).
And this is the fun part. I took these pics today - just 8 weeks after the above photo, the clipped part that was pale is now dark (summer coat) and the hairy part that was dark (winter coat) is now white! Amazing colour changing ponies