I notice orange, cream-gold and red colored pollen being brought into the hive.
Should I feed with dry sugar or a candy board instead of open feeding with syrup?
I think the red pollen is henbit. If your bees have enough stores, don't worry. But if the warm weather continues and your hives are using up all their stores, you have to feed something. I think it's too late to feed syrup. They won't take it if the syrup temperature is below 50F, so you can't leave the syrup out overnight and expect it to get warmer than 50F during the day, even if the maximum air temp is 60F. Also, in my opinion 100 feet is too close to the hive to be feeding syrup. The bees use a "round dance" out to 75 yards. This dance does not provide enough direction and distance info so the bees end up searching the area around your hives for the scent of the syrup. This can trigger robbing of one hive by another. I feed at 100 yds and this does not seem to start robbing behavior, I guess because the bees are doing a waggle dance that sends the foragers away from the hive. Anyway, that's my story and I'm sticking with it. :-D
You could try feeding warmed syrup, either open or in a top feeder. But of course the bees will only be able to take a small amount of it before it cools down too much. And feeding syrup this time of year increases the humidity in the hive since the bees don't have time to dry and cap it before real cold weather starts. High humidity is not good for bees in winter.
So if you must feed, the best thing is dry sugar. If you add a box (we use shallows) above the brood nest and put newspaper over the brood frames, you can put a pile of sugar (say 5 lbs) on the newspaper. This is easy to check and periodically refill if necessary during the winter. I pour a little water on the sugar so it will clump and not fall out of the hive (we have screened bottoms) or be tossed out by the bees as trash. Only one of my hives has dry sugar on it right now. I checked yesterday and a small clump of bees, maybe 20 bees in all, was sitting next to the pile munching away on the sugar. I leave a gap between the newspaper and the edge of the box on one side so the bees can easily move between the sugar box and the broodnest.