Board Thread:Questions and Answers/@comment-26952946-20160609150654/@comment-27701762-20160629022238

I'm not sure how much we can extrapolate from the soul modes we see in the game. Each change is temporary, and forced upon the player through some other means. This alteration is especially relevant for the Green and Yellow soul modes, since Undyne says she gives you a spear to block with, and it is remarked on how she "turns people's hearts green," while the yellow mode is activated by a special feature that Alphys builds into your cell phone. Then of course you have the "blue attack" from Papyrus and Sans, and Muffet's purple liquid which traps you on the lines (not unlike moving between strings on a spider web). Each soul mode seems to be heavily sensitive to the specific context the player is in.

It may instead be that any soul can take on any sort of battle mode, and that certain colors correspond to the mode, rather than the way they "normally" fight. Put another way, in controlling the Green-souled human, we would use the Red soul mode most of the time, unless the context of the battle forced us into the Green soul mode.

Another possibility is that there is no proper "Red soul mode," but that it is perceived to be such because that is the soul we play with most of the time. So that if the game were played with the Green-souled human instead, nothing would change except that the soul's color would not change during the Undyne fight.