I used color smart for men medium brown on my hair and my french lace human hairpiece, it is too dark and i want to use the light brown but am not sure what to do for both my hair and the hairpiece

You may want to use the lighter shade and re-dye it, or simply show it more sunlight. The light from the sun will naturally brighten your hair.

Whenever you go from a very light color to a dark color, you need a "filler" to make the dark color look right. The med brown color you used can act as your filler. Wait a couple of days, then put another med brown color on your hair. If your hair is is good condition, that should take care of your problem. If that is still too light or too gray, use a temporary color (one that lasts 6-8 shampoos). These are not so harsh on your hair and can be applied more frequently until your hair is the color you want. Once you get it to the color you want, you can just apply the med brown when it is time to color your hair again.

Just make sure you wait a few days between treatments, because your hair will become too porous if you color it too frequently and it will become damaged. There are other types of fillers you can purchase from a beauty supply store or website (like Sally's).

If you have a beauty supply store near you, like Sally's, purchase Roux Fanci Full Styling Mousse in one of the blonde shades to tone down the red. This product works really well and provides a wide variety of colors,

Hi no you wont be able to put another colour on top, you will have to wait till it fades. If you put another colour on top you will get colour build up!