filming iridescent liquid or effect
What you are asking is not quite clear, but if you want to have the colored liquids in front of your people, just add the video on top of the main video, and just change the opacity of the colored liquids. If you want the iridescent liquids behind your people, shoot the main video in front of a green screen with fairly good lighting, then add the colored liquid in after, utilizing the green screen. 

Hope this helps!