In page 19 of Just Cause, I had two such panels, so today I'm sharing them in all their full-sized, unobstructed glory.
The Heroine Appears
Most fan art I've seen depicts Undyne's Undying armor mostly in black or dark grey, and I did the same the first time I drew her. But this time, I wanted to add in some of her friends' colors, to show that she was thinking of them when she created this form for herself.
The gold on her belt and gloves is the same shade as Asgore's hair and shoulder guards, and the hearts on her gloves are Alphys' yellow rimmed with the orange of Papyrus' scarf.
The red in her armor symbolizes the determination that enabled her to drag herself back from the brink of death for one last glorious fight.
Please Don't Noogie The Skeleton!
Judging by her snow wrestling tendencies, Undyne and other people's boundaries are not the best combination. But Papyrus will always forgive her and go right on being her cool friend, after he's done yelling in futile protest.
And then he will go and pass the noogie on to his brother, in a much angstier piece of fan art.