When you use a layer as a track matte you create an alpha channel on the layer below so there is transparency. The color values of the alpha channel when you choose Alpha Trac Matte or the color values of the RGB channels when you choose Luma Track Matte, control the opacity.
If you put a solid on top of another layer and it doesn't cover the entire frame then you can use that layer as an alpha track matte for the layer below to add some transparency. Then you can turn the layer back on and set a blend mode to blend the color of the solid layer with the layer below. The transparency from the alpha channel (the transparent part) of the solid layer will still create transparency on the layer below and the pixels will blend with the layer below.
You can do the same thing using luma matte but if the color of the solid layer is anywhere near black or white the blend modes will not be as effective.
I have a strong feeling that you are either are not explaining your project correctly or you are using a real sideways approach to solving the problem of creating a shadow on your layer. It would really help if you included a screenshot with the modified properties of the layers you are working with revealed. Just press the U key twice, PrintScreen and paste to the forum and then give us a clear explanation of what you are trying to accomplish.