Light brown and light ash brown hair color are bascily the same color, they are only diffrent because of the tone of the color. The tone of the color is name "ash". Light Ash Brown Hair Color will help you neutralize any tones in your hair like yellow or gold ones. This will help you if you have any brassiness from your previous colorings, then ash tone can help you, a light brown may not.
This is the main difference between light brown and light ash brown hair color.