Wow thanks!! I feed my horses sunflower seeds, but I guess cause I've fed them that for so long I haven't noticed the difference in coat!! I'll be introducing the sunflower seeds in Mim's (new horse) feed and ill have to keep a eye out of any change! (: I'll take some before and after photos :P
Another thing to consider (just saw you're in australia too so similar product lines to the ones in my area) is livamol... it is expensive but you don't use much at a time. It turned my friend's brown from almost bay to literally black, the only way you can tell he's brown is because his muzzle stayed light. I don't use it because I have a bright bay (whose colour I love the way it is) and a buckskin, whose colour I love even more than the bay's.
The biochemical pathway which produces both pigments involves an enzyme that uses copper ions as an electron acceptor. Thus for the pigments to be produced fully there needs to be copper present in the cell.
If the horse is short of copper then the enzyme can't do it's job and the pigments are produced slowly if at all and sometimes in a different form.
So to ensure that the animal gets it's full dose of pigment synthsized you need to ensure a supply of copper in the diet.
Paprika is rich in copper flavenoids, especially the Hungarian paprika.
Thus as long as you feed a supplement rich in copper the hairs will get their full dose of pigment. BUT this only happens when the hairs are being produced as they grow, once they are grown, you cannot add any more pigment, you can only either lose it to the action of sunlight and or chemicals.
This is by an article by Melyni (PhD) PAS, Dipl. ACAN.
Ah yes there's a thing in peppers (and therefore paprika) that makes them hot, that's banned for most shows. I know what it's called, I just can't figure out how to spell it :/ might spell it out phonetically.
Cap-sigh-a-sin. Can't spell it. But that's how it's said.
Edit; but sunny seeds aren't swabbable :) like I said there's also livamol. Do you use lucerne chaff and/or hay? That darkens too. (I don't know if those US members have something similar, lucerne is dark green with a purple flower and it smells really sweet. Horses LOVE it. OP is australian though and lucerne is a very popular feed here in aus)