I'm against choosing a different color for every different sprite. I do, however, think that it would be nice to have the possibility for a
sprite to have the title's color, and vice-versa. The reason is simple: help people making side-scrolling games!
Currently you can't really pass through a sprite, and I fully understand why - you interact with a sprite by bumping on it, so passing through would always trigger a dialogue.
At the moment, you can circumvent the problem by making the NPC an animated title and put an invisible sprite over it - so you can trigger the conversation if you press UP in front of it. But this means that "walkable" sprites will always have the same color of the title, and it can look confusing.
(Hope my ramblings are clear. I can't English at 2AM.)