Found some research
Sorry it's taken a while but I've looked through my sheltie Nutshell magazines and found the following for you, printed in 2003 as a result of a European wide discussion on tri-colours going rusty:
Dutch breeders discussion:
None of the Dutch breeders though sunshine was a contributary factor into tri colour rusting. Coat changing was mentioned as a major factor in the tendancy to rust (something I've noticed too).
Food was mentioned, some brands can affect the coat colour (possibly dry food has been mentioned) and too many carrots. (Mine have carrots in food and as treats, but I don't think it has a massive impact personally) Carotene is apparently added to the fod of show canaries to get the red colouring.
Some coat condtioners may also have a negative effect.
Genetics, certainly a major factor. If there are rusty tris in the background it is more likely to come out.
Germany breeders:
Very similar to the above but sunlight was again mentioned, also 'dry' coats being more prone to going rusty. Possibly as a result of hot summer/heating?
Seeweed crops up several times but confusingly some say it causes the red, some say prevents, so the jury is out on this one.
The british breeders mostly said genetics and sunlight were contributary factors.
One mentioned a tri going rusty after spot on flea treatment.
Possible suggestions from the magazine:
bath and brush vigoursly within a couple of days to ensure all the dead hair is removed and use a black coat shampoo. Possibly a coat conditioner with UV protection could help.
I think I've got it all down more or less, though a bit paraphrased! Hope this helps or is at leastof some interest.
