So apparently Hyakkimaru and Biwamaru see white to a soul which is pure, and see red the soul of someone who has killed other humans or monsters who are evil. Are there other colour meanings?
There is just gray, that is a normal human, White means a human with a pure soul, red means a ghoul or a demon you see it for the first time in episode 1. gray with red lines means a human who killed people it's first seen in episode 5. In episode 12 you see a green soul. This from the statue (the god). Green is probarly wood. In episode 16 you see that a tree also has a green soul and the right arm of hyakimaru is also green.
In episode 13, there is 1 new color,
a soul that was gray with black holes, this was a human that was revived from the death by a demon.
In episode 14, there are 2 new colors,
the color blue and yellow. If I have to guess those are ghosts that are killed people.