How can I generate the opposite color according to current color?

I'm trying to create a color opposite of current color. I mean if current color is black, then I need to generate white.

Actually I have a text (the color of this text is dynamic, its color can be made at random). That text is into a div and I need to set the opposite color of that text for the background-color of div. I would like that text be clear in the div(color perspective).

The opposite color means: Dark / Bright

I have the current color of text and I can pass it to this function:

var TextColor = #F0F0F0;    // for example (it is a bright color)

function create_opp_color(current color) {

    // create opposite color according to current color

}

create_opp_color(TextColor); // this should be something like "#202020" (or a dark color)

Is there any idea to create create_opp_color() function?