So first, I'm going to drag my main clip to the timeline, then my overlaying clip to the timeline, but in a layer above the main clip. Just like this: http://prntscr.com/2oox26

Then I'm going to click the Video FX tab, then find and click Chroma Keyer, then drag "Default" to my overlaying clip. Just like this: http://prntscr.com/2oouw6

When the effects window pops up, you're going to click the little triangle to bring down more color options, click the color selection tool, and then choose the color that you want to get rid of. Just like this: http://prntscr.com/2ooyhv

You will notice that the color will disappear, but your image may be a little transparent. To fix this, you are going to edit your low and high threshold values. I recommend changing the high threshold first, then changing the low threshold for some finer details. Just like this: http://prntscr.com/2oozow

I hope this helps.