In that case, set it to zero, and then for the homepage change the transparent part of the Additional CSS code for any color you like.
The color can be hexadecimal, rgb or rgba, for example #ff0000, rgb(255,0,0) or rgba(255,0,0,1)
in the rgba example the 1 at the end controls the transparency (0 is fully transparent and 1 is no transparency, so 0.5 would be half transparent).