Since we started using Foal Response at birth (and deworming the dam at foaling too) we've only had one case of foal diarrhea (this year out of three years so far) and that was a foal that nibbles on everything! She's had two cases of it so far but no fever, is bright, nurses and plays so I'm assuming it's her tendency to taste things that don't agree. But as far as 'color' goes its just liquified normal poop.
The only thing I would be careful of when it comes to diarrhea is the really nasty yellow/brownish/orange color smelly one - we call that the death diarrhea here as foals go down fast and crash from it. It can be a rotavirus, or septic problems or some other serious problem - all require fast veterinarian intervention and generally IV fluids as well as immune system support.
As for weight on older mares - definitely they should be de-wormed (we do this the first 12 hours after foaling - another help in fighting off foal heat diarrhea), and if the teeth look good - then up the feed! If you're expecting a mare to produce quality milk, prepare her body for another foal and meet her own nutritional needs you really need to be providing her excellent forage and possibly a great grain supplement as well.
We feed alfalfa 2x a day with a bale of high quality grass hay available 24/7 - we go for the softest grass hay for the foals that like to test grazing and for the mares that are in need. Nutrena Senior is a great feed for minis and we supplement mares and foals with that as well. And yes I do have a very 'fluffy' herd of horses - but they're shiny-glossy, alert, happy and have excellent fertility so they can carry a few extra pounds - I hate to see skinny mares trying to grow a foal - it's cruel IMHO!
I have one mare that is 19 and if she drops her weight this year I won't rebred her its just taking to much out of her. She came out of last year thinner than I'd like and we've been free choice feeding her since - she's naturally a lean mare so keeping weight on her is a bit of work - she's not like the rest of the chunkers here!