What do the colours of the souls mean in Dororo?
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.