I gave them a couple tips, and decided to share those tips here, too.
1. When a dark-colored light source is casting light on a non-black object, especially one with a lighter shade than the light source, make the color of the glow slightly lighter than the color of its source.
When the light source's color is darker than the object on which it's casting light (like the soul compared to the shirt), it makes it look like the soul is casting pure color or even darkness on the object it's glowing on.
2. Add a lighter shade of the base color to the center of the soul, so it looks more like a light source.
Here's a quick demo of what the abovementioned glow effects look like in total darkness and on clothing the color of the shirt they drew, so you can see the difference.
The souls on the bottom are glowing their base color in the dark, but on the shirt, the glow is lighter than the base color, so they aren't darkening the shirt.
I hope that quick art tip helps!