I have two layers. One is a background image and the other is a postscript. The postscript is text in various colors. The problem I have is when the postscript text is the same color as the background image. Can I add another layer of the same postscript and invert all of its colors with one command?

There's a command to invert colours (Effects -> Color -> Negative). This is somewhat limited in what it can handle, but it's worth a try.

It would also be possible to invert colours using a filter.

I created a negative overlay filter and uploaded it to Open Clip Art. When applied to an object, the colors of the object will render as the negative of whatever is underneath it. Here's an example, with the filter applied to the text object:

I have no color in my efects menu. I am in version Inkscape 0.44.1, built Sep 25 2006 Debian Etch.

I used a text editor and copied the filter <filter
inkscape:label="negative overlay"
id="filter3187">
<feColorMatrix
in="BackgroundImage"
values="-1 0 0 0 1 0 -1 0 0 1 0 0 -1 0 1 0 0 0 1 0 "
id="feColorMatrix3189" />
<feComposite
operator="in"
in2="SourceGraphic"
id="feComposite3191" />
</filter>
into the def section by hand I dont think this version has alot of features you guys are talking about.

That's right, there's no support for filters pre-0.46 (except blur in 0.45).