I'm not sure specifically how it works, but there's this color mixture technique in Adobe Photoshop when you take a grayscale image and change the mode to duotone or tritone. You select a few colors, and it combines them all...cool like. I'm sorry, I'm not sure exactly how to describe it.

Anyhoo, if anyone knows what I'm talking about, is there a way to do this in final cut pro? I'm using 4.1.

It's not as easy as just applying a filter, but here's one way to do it:
Take your clip and apply these filters in this order: Image control>Desaturate , Color Correction>3-way Color Corrector , and a second Color Correction>3-way Color Corrector. Desaturate amount should be 100. For the first 3-way CC, apply the Luma limit effect and set the Minimum to 0 and have the width going about half-way with a good amount of softness. This is your dark color. Adjust the color using the three color wheels and the level using the sliders underneath each color wheel. Now go to your second 3-way CC filter. On this one, set the Luma Limit effect max all the way at 100 and the minimum at about halfway with a good amount of softness. This is your brights color. Use the color wheels and sliders to adjust levels and color. Now if you want to do three colors, just add another 3-way CC filter and create narrower Luma limit effect areas on each 3-way CC filter--one for lows, one for mids, and one for the highs. You'll have a ton of control on the effect--just play with the sliders and the luma limit controls.

hope this was clear. best luck.