rat, mouse, dark gray
鼡 centers on a small rodent: a rat or mouse. The dark gray sense comes from the typical color of these animals.
鼡 is a variant form of 鼠 (mouse/rat). The exact historical development of this simplified shape is uncertain, but it likely derives from a cursive or abbreviated version of the more complex 鼠.
The top part ⺌ looks like a small snout and ears, while the lower part 用 resembles a body with a tail. Together they form a mouse or rat.
For ソ, imagine a mouse nibbling on a piece of sewing thread: sew -> ソ, and the mouse is busy with the thread.