Underfell is where, instead of everyone being nice, kind and good-hearted, they are evil and try to kill the character throughout their journey* **. I, however, agree with the idea that they each have a good side, and the goal of the game is to bring each of the characters back to normal by bringing out each of these qualities: for example, Sans' quality is care, as he cares very much for his brother, and would give his life to protect him. Papyrus' quality is fulfilment, in others' or his own eyes; if he feels like he's achieved an enormous goal, or redeemed himself from something awful in someone else's eyes, he would come to his senses. Also, the colour palette is white, black red and yellow.
*And/or they have opposite personality traits, for example, Monster Kid despises the help of anyone,/ generally hates them, and hates pity. He is very bitter and hot-headed. (Basically the general opposite of his sweet, kind-of annoying undertale self.)
** Flowey, instead of being a soulless beast, and Chara, instead of being a murderous pyromaniac, are sad, kind beings that gave up on the actually Kill or Be Killed world, and so Asriel is evil instead of the latter.