Grapes are great! I'd maybe switch it around a little more and pull back on the grapes. They can be high in something that for some reason is now totally blanking from my mind and can build up in their little bodies. I feed them grapes as treats and snacks regularly, but try for more apple for a bit and see if it changes up their #2's and makes them a consistent brownish color.
off color noses usually are purpleish. If no purple then yay! Every glider has a different nose, just like people, so it can be hard to say "this specific color is unhealthy" if they get cold their noses can be darker... But one of my gliders if especially sleepy and slow moving never has a bright pink nose until her heart rate rises, so sometimes it's just trial and error!
Link and Zelda are adorable names, by the way.
for the grape thing, don't freak out. food builds up a lot of whatever it has a lot of. Keeping a hefty variety can be tough sometimes! Don't stress....
I'd just keep working with them. Since you haven't seen any strange discoloration or any off type of anything else I'd say watch them and love em and listen and notice what sounds are happening when (which you're doing already yay!!) and keep getting to know them!! Gliders. Make. noise. I used to freak out and think something was wrong with them before I found out the sound they make when they clean themselves is literally just them spitting on their hands.. Then I SAW them do it and calmed down forever. Things happen! Keep up the good work!